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HTTP/1.1, part 2: Semantics and PayloadsAdobe Systems Incorporated345 Park AveSan JoseCA95110USAfielding@gbiv.comhttp://roy.gbiv.com/World Wide Web ConsortiumW3C / ERCIM2004, rte des LuciolesSophia-AntipolisAM06902Franceylafon@w3.orghttp://www.raubacapeu.net/people/yves/greenbytes GmbHHafenweg 16MuensterNW48155Germanyjulian.reschke@greenbytes.dehttp://greenbytes.de/tech/webdav/HTTPbis Working Group
The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
distributed, collaborative, hypertext information systems. This document
defines the semantics of HTTP/1.1 messages, as expressed by request
methods, request header fields, response status codes, and response header
fields, along with the payload of messages (metadata and body content) and
mechanisms for content negotiation.
Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at
.
The current issues list is at
and related
documents (including fancy diffs) can be found at
.
The changes in this draft are summarized in .
Each HTTP message is either a request or a response. A server listens on a
connection for a request, parses each message received, interprets the
message semantics in relation to the identified request target, and
responds to that request with one or more response messages.
This document defines HTTP/1.1 request and response semantics in terms of
the architecture, syntax notation, and conformance criteria defined in
.
HTTP provides a uniform interface for interacting with resources regardless
of their type, nature, or implementation. HTTP semantics includes the
intentions defined by each request method, extensions to those semantics
that might be described in request header fields, the meaning of status
codes to indicate a machine-readable response, and additional control data
and resource metadata that might be given in response header fields.
In addition, this document defines the payload of messages (a.k.a.,
content), the associated metadata header fields that define how the payload
is intended to be interpreted by a recipient, the request header fields
that might influence content selection, and the various selection
algorithms that are collectively referred to as
"content negotiation".
&Note; This document is currently disorganized in order to minimize changes
between drafts and enable reviewers to see the smaller errata changes.
A future draft will reorganize the sections to better reflect the content.
In particular, the sections will be ordered according to the typical
processing of an HTTP request message (after message parsing): resource
mapping, methods, request modifying header fields, response status,
status modifying header fields, and resource metadata. The current mess
reflects how widely dispersed these topics and associated requirements
had become in .
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in .
This specification targets conformance criteria according to the role of
a participant in HTTP communication. Hence, HTTP requirements are placed
on senders, recipients, clients, servers, user agents, intermediaries,
origin servers, proxies, gateways, or caches, depending on what behavior
is being constrained by the requirement. See &architecture; for definitions
of these terms.
The verb "generate" is used instead of "send" where a requirement
differentiates between creating a protocol element and merely forwarding a
received element downstream.
An implementation is considered conformant if it complies with all of the
requirements associated with the roles it partakes in HTTP. Note that
SHOULD-level requirements are relevant here, unless one of the documented
exceptions is applicable.
This document also uses ABNF to define valid protocol elements
().
In addition to the prose requirements placed upon them, senders &MUST-NOT;
generate protocol elements that do not match the grammar defined by the
ABNF rules for those protocol elements that are applicable to the sender's
role. If a received protocol element is processed, the recipient &MUST; be
able to parse any value that would match the ABNF rules for that protocol
element, excluding only those rules not applicable to the recipient's role.
Unless noted otherwise, a recipient &MAY; attempt to recover a usable
protocol element from an invalid construct. HTTP does not define
specific error handling mechanisms except when they have a direct impact
on security, since different applications of the protocol require
different error handling strategies. For example, a Web browser might
wish to transparently recover from a response where the
Location header field doesn't parse according to the ABNF,
whereas a systems control client might consider any form of error recovery
to be dangerous.
This specification uses the Augmented Backus-Naur Form (ABNF) notation
of with the list rule extension defined in
&notation;. describes rules imported from
other documents. shows the collected ABNF
with the list rule expanded.
The method token indicates the request method to be performed on the target
resource (&effective-request-uri;). The method is case-sensitive.
method = token
The list of methods allowed by a resource can be specified in an
Allow header field (). The
status code of the response always notifies the client whether a method is
currently allowed on a resource, since the set of allowed methods can change
dynamically. An origin server &SHOULD; respond with the status code
405 (Method Not Allowed) if the method is known by the origin
server but not allowed for the resource, and 501 (Not
Implemented) if the method is unrecognized or not implemented by the
origin server. The methods GET and HEAD &MUST; be supported by all
general-purpose servers. All other methods are &OPTIONAL;; however, if the
above methods are implemented, they &MUST; be implemented with the same
semantics as those specified in .
Implementers need to be aware that the software represents the user in
their interactions over the Internet, and need to allow
the user to be aware of any actions they take which might have an
unexpected significance to themselves or others.
In particular, the convention has been established that the GET, HEAD,
OPTIONS, and TRACE request methods &SHOULD-NOT; have the significance
of taking an action other than retrieval. These request methods ought
to be considered "safe".
This allows user agents to represent other methods, such as POST, PUT
and DELETE, in a special way, so that the user is made aware of the
fact that a possibly unsafe action is being requested.
Naturally, it is not possible to ensure that the server does not
generate side-effects as a result of performing a GET request; in
fact, some dynamic resources consider that a feature. The important
distinction here is that the user did not request the side-effects,
so therefore cannot be held accountable for them.
Request methods can also have the property of "idempotence" in that,
aside from error or expiration issues, the intended effect of multiple
identical requests is the same as for a single request.
PUT, DELETE, and all safe request methods are idempotent.
It is important to note that idempotence refers only to changes
requested by the client: a server is free to change its state due
to multiple requests for the purpose of tracking those requests,
versioning of results, etc.
The HTTP Method Registry defines the name space for the method token in the
Request line of an HTTP request.
Registrations &MUST; include the following fields:
Method Name (see )Safe ("yes" or "no", see )Idempotent ("yes" or "no", see )Pointer to specification text
Values to be added to this name space require IETF Review
(see ).
The registry itself is maintained at .
When it is necessary to express new semantics for a HTTP request that
aren't specific to a single application or media type, and currently defined
methods are inadequate, it might be appropriate to register a new method.
HTTP methods are generic; that is, they are potentially applicable to any
resource, not just one particular media type, "type" of resource, or
application. As such, it is preferred that new HTTP methods be registered
in a document that isn't specific to a single application, so that this is
clear.
Due to the parsing rules defined in &message-body;, definitions of HTTP
methods cannot prohibit the presence of a message body on either the request
or the response message (with responses to HEAD requests being the single
exception). Definitions of new methods cannot change this rule, but they can
specify that only zero-length bodies (as opposed to absent bodies) are allowed.
New method definitions need to indicate whether they are safe (), what semantics (if any) the request body has,
and whether they are idempotent ().
They also need to state whether they can be cached (&caching;); in
particular under what conditions a cache can store the response, and under
what conditions such a stored response can be used to satisfy a subsequent
request.
yesyes
The OPTIONS method requests information about the
communication options available on the request/response chain
identified by the effective request URI. This method allows a client to
determine the options and/or requirements associated with a resource,
or the capabilities of a server, without implying a resource action
or initiating a resource retrieval.
Responses to the OPTIONS method are not cacheable.
If the OPTIONS request includes a message body (as indicated by the
presence of Content-Length or Transfer-Encoding),
then the media type &MUST; be indicated by a Content-Type
field. Although this specification does not define any use for such a body,
future extensions to HTTP might use the OPTIONS body to make more detailed
queries on the server.
If the request-target (&request-target;) is an asterisk ("*"),
the OPTIONS request is
intended to apply to the server in general rather than to a specific
resource. Since a server's communication options typically depend on
the resource, the "*" request is only useful as a "ping" or "no-op"
type of method; it does nothing beyond allowing the client to test
the capabilities of the server. For example, this can be used to test
a proxy for HTTP/1.1 conformance (or lack thereof).
If the request-target is not an asterisk, the OPTIONS request applies
only to the options that are available when communicating with that
resource.
A 200 (OK) response &SHOULD; include any header fields that
indicate optional features implemented by the server and applicable to that
resource (e.g., Allow), possibly including extensions not
defined by this specification. The response body, if any, &SHOULD; also
include information about the communication options. The format for such a
body is not defined by this specification, but might be defined by
future extensions to HTTP. Content negotiation &MAY; be used to select
the appropriate response format. If no response body is included, the
response &MUST; include a Content-Length field with a
field-value of "0".
The Max-Forwards header field &MAY; be used to target a
specific proxy in the request chain (see ).
If no Max-Forwards field is present in the request, then the forwarded
request &MUST-NOT; include a Max-Forwards field.
yesyes
The GET method requests transfer of a current representation of
the target resource.
If the target resource is a data-producing process, it is the
produced data which shall be returned as the representation in the response and not
the source text of the process, unless that text happens to be the output of
the process.
The semantics of the GET method change to a "conditional GET" if the
request message includes an If-Modified-Since,
If-Unmodified-Since, If-Match,
If-None-Match, or If-Range header field
(). A conditional GET requests that the representation
be transferred only under the circumstances described by the conditional
header field(s). The conditional GET request is intended to reduce
unnecessary network usage by allowing cached representations to be refreshed
without requiring multiple requests or transferring data already held by the
client.
The semantics of the GET method change to a "partial GET" if the request
message includes a Range header field ().
A partial GET requests that only part of the representation be transferred,
as described in &header-range;. The partial GET request is intended to reduce
unnecessary network usage by allowing partially-retrieved representations to
be completed without transferring data already held by the client.
Bodies on GET requests have no defined semantics. Note that sending a body
on a GET request might cause some existing implementations to reject the
request.
The response to a GET request is cacheable and &MAY; be used to satisfy
subsequent GET and HEAD requests (see &caching;).
See for security considerations when used for forms.
yesyes
The HEAD method is identical to GET except that the server &MUST-NOT;
return a message body in the response. The metadata contained
in the HTTP header fields in response to a HEAD request &SHOULD; be identical
to the information sent in response to a GET request. This method can
be used for obtaining metadata about the representation implied by the
request without transferring the representation body. This method is
often used for testing hypertext links for validity, accessibility,
and recent modification.
The response to a HEAD request is cacheable and &MAY; be used to satisfy
a subsequent HEAD request. It also has potential side effects on
previously stored responses to GET; see &p6-head;.
Bodies on HEAD requests have no defined semantics. Note that sending a body
on a HEAD request might cause some existing implementations to reject the
request.
The POST method requests that the origin server accept the
representation enclosed in the request as data to be processed by the
target resource. POST is designed to allow a uniform method to cover the
following functions:
Annotation of existing resources;
Posting a message to a bulletin board, newsgroup, mailing list,
or similar group of articles;
Providing a block of data, such as the result of submitting a
form, to a data-handling process;
Extending a database through an append operation.
The actual function performed by the POST method is determined by the
server and is usually dependent on the effective request URI.
The action performed by the POST method might not result in a resource that
can be identified by a URI. In this case, either 200 (OK) or
204 (No Content) is the appropriate response status
code, depending on whether or not the response includes a representation
that describes the result.
If a resource has been created on the origin server, the response
&SHOULD; be 201 (Created) and contain a representation which
describes the status of the request and refers to the new resource, and a
Location header field (see ).
Responses to POST requests are only cacheable when they
include explicit freshness information (see &p6-explicit;). A
cached POST response with a Content-Location header field
(see &header-content-location;) whose value is the effective
Request URI &MAY; be used to satisfy subsequent GET and HEAD requests.
Note that POST caching is not widely implemented.
However, the 303 (See Other) response can be used to direct
the user agent to retrieve a cacheable representation of the resource.
yes
The PUT method requests that the state of the target resource
be created or replaced with the state defined by the representation
enclosed in the request message payload. A successful PUT of a given
representation would suggest that a subsequent GET on that same target
resource will result in an equivalent representation being returned in
a 200 (OK) response. However, there is no guarantee that
such a state change will be observable, since the target resource might be
acted upon by other user agents in parallel, or might be subject to dynamic
processing by the origin server, before any subsequent GET is received.
A successful response only implies that the user agent's intent was
achieved at the time of its processing by the origin server.
If the target resource does not have a current representation and
the PUT successfully creates one, then the origin server &MUST; inform
the user agent by sending a 201 (Created) response. If the
target resource does have a current representation and that representation is
successfully modified in accordance with the state of the enclosed
representation, then either a 200 (OK) or 204 (No Content)
response &SHOULD; be sent to indicate successful completion of the request.
Unrecognized header fields &SHOULD; be ignored (i.e., not saved
as part of the resource state).
An origin server &SHOULD; verify that the PUT representation is
consistent with any constraints which the server has for the target
resource that cannot or will not be changed by the PUT. This is
particularly important when the origin server uses internal
configuration information related to the URI in order to set the
values for representation metadata on GET responses. When a PUT
representation is inconsistent with the target resource, the origin
server &SHOULD; either make them consistent, by transforming the
representation or changing the resource configuration, or respond
with an appropriate error message containing sufficient information
to explain why the representation is unsuitable. The
409 (Conflict) or 415 (Unsupported Media Type)
status codes are suggested, with the latter being specific to constraints on
Content-Type values.
For example, if the target resource is configured to always have a
Content-Type of "text/html" and the representation being PUT
has a Content-Type of "image/jpeg", then the origin server &SHOULD; do one of:
reconfigure the target resource to reflect the new media type;transform the PUT representation to a format consistent with that
of the resource before saving it as the new resource state; or,reject the request with a 415 (Unsupported Media Type)
response indicating that the target resource is limited to "text/html",
perhaps including a link to a different resource that would be a
suitable target for the new representation.
HTTP does not define exactly how a PUT method affects the state
of an origin server beyond what can be expressed by the intent of
the user agent request and the semantics of the origin server response.
It does not define what a resource might be, in any sense of that
word, beyond the interface provided via HTTP. It does not define
how resource state is "stored", nor how such storage might change
as a result of a change in resource state, nor how the origin server
translates resource state into representations. Generally speaking,
all implementation details behind the resource interface are
intentionally hidden by the server.
The fundamental difference between the POST and PUT methods is
highlighted by the different intent for the target resource.
The target resource in a POST request is intended to handle the
enclosed representation as a data-accepting process, such as for
a gateway to some other protocol or a document that accepts annotations.
In contrast, the target resource in a PUT request is intended to
take the enclosed representation as a new or replacement value.
Hence, the intent of PUT is idempotent and visible to intermediaries,
even though the exact effect is only known by the origin server.
Proper interpretation of a PUT request presumes that the user agent
knows what target resource is desired. A service that is intended
to select a proper URI on behalf of the client, after receiving
a state-changing request, &SHOULD; be implemented using the POST
method rather than PUT. If the origin server will not make the
requested PUT state change to the target resource and instead
wishes to have it applied to a different resource, such as when the
resource has been moved to a different URI, then the origin server
&MUST; send a 301 (Moved Permanently) response; the user
agent &MAY; then make its own decision regarding whether or not to redirect
the request.
A PUT request applied to the target resource &MAY; have side-effects
on other resources. For example, an article might have a URI for
identifying "the current version" (a resource) which is separate
from the URIs identifying each particular version (different
resources that at one point shared the same state as the current version
resource). A successful PUT request on "the current version" URI might
therefore create a new version resource in addition to changing the
state of the target resource, and might also cause links to be added
between the related resources.
An origin server &SHOULD; reject any PUT request that contains a
Content-Range header field (&header-content-range;), since
it might be misinterpreted as partial content (or might be partial content
that is being mistakenly PUT as a full representation). Partial content
updates are possible by targeting a separately identified resource with
state that overlaps a portion of the larger resource, or by using a
different method that has been specifically defined for partial
updates (for example, the PATCH method defined in
).
Responses to the PUT method are not cacheable. If a PUT request passes
through a cache that has one or more stored responses for the effective
request URI, those stored responses will be invalidated (see
&p6-invalid;).
yes
The DELETE method requests that the origin server delete the target
resource. This method &MAY; be overridden by
human intervention (or other means) on the origin server. The client cannot
be guaranteed that the operation has been carried out, even if the
status code returned from the origin server indicates that the action
has been completed successfully. However, the server &SHOULD-NOT;
indicate success unless, at the time the response is given, it
intends to delete the resource or move it to an inaccessible
location.
A successful response &SHOULD; be 200 (OK) if the response
includes a representation describing the status, 202 (Accepted)
if the action has not yet been enacted, or 204 (No Content) if
the action has been enacted but the response does not include a representation.
Bodies on DELETE requests have no defined semantics. Note that sending a body
on a DELETE request might cause some existing implementations to reject the
request.
Responses to the DELETE method are not cacheable. If a DELETE request
passes through a cache that has one or more stored responses for the
effective request URI, those stored responses will be invalidated (see
&p6-invalid;).
yesyes
The TRACE method requests a remote, application-layer loop-back
of the request message. The final recipient of the request
&SHOULD; reflect the message received back to the client as the message body
of a 200 (OK) response. The final recipient is either the
origin server or the first proxy to receive a Max-Forwards
value of zero (0) in the request (see ).
A TRACE request &MUST-NOT; include a message body.
TRACE allows the client to see what is being received at the other
end of the request chain and use that data for testing or diagnostic
information. The value of the Via header field (&header-via;)
is of particular interest, since it acts as a trace of the request chain.
Use of the Max-Forwards header field allows the client to
limit the length of the request chain, which is useful for testing a chain of
proxies forwarding messages in an infinite loop.
If the request is valid, the response &SHOULD; have a
Content-Type of "message/http" (see &media-type-message-http;)
and contain a message body that encloses a copy of the entire request message.
Responses to the TRACE method are not cacheable.
The CONNECT method requests that the proxy establish a tunnel
to the request-target and, if successful, thereafter restrict its behavior
to blind forwarding of packets until the connection is closed.
When using CONNECT, the request-target &MUST; use the authority form
(&request-target;); i.e., the request-target consists of only the
host name and port number of the tunnel destination, separated by a colon.
For example,
CONNECT server.example.com:80 HTTP/1.1
Host: server.example.com:80
Any 2xx (Successful) response to a CONNECT request indicates that the
proxy has established a connection to the requested host and port,
and has switched to tunneling the current connection to that server
connection.
The tunneled data from the server begins immediately after the blank line
that concludes the successful response's header block.
A server &SHOULD-NOT; send any Transfer-Encoding or
Content-Length header fields in a successful response.
A client &MUST; ignore any Content-Length or Transfer-Encoding header
fields received in a successful response.
Any response other than a successful response indicates that the tunnel
has not yet been formed and that the connection remains governed by HTTP.
Proxy authentication might be used to establish the
authority to create a tunnel:
CONNECT server.example.com:80 HTTP/1.1
Host: server.example.com:80
Proxy-Authorization: basic aGVsbG86d29ybGQ=
A message body on a CONNECT request has no defined semantics. Sending a
body on a CONNECT request might cause existing implementations to reject
the request.
Similar to a pipelined HTTP/1.1 request, data to be tunneled from client
to server &MAY; be sent immediately after the request (before a response
is received). The usual caveats also apply:
data can be discarded if the eventual response is negative, and the
connection can be reset with no response if more than one TCP segment
is outstanding.
It might be the case that the proxy itself can only reach the requested
origin server through another proxy. In this case, the first proxy
&SHOULD; make a CONNECT request of that next proxy, requesting a tunnel
to the authority. A proxy &MUST-NOT; respond with any 2xx status code
unless it has either a direct or tunnel connection established to the
authority.
If at any point either one of the peers gets disconnected, any
outstanding data that came from that peer will be passed to the other
one, and after that also the other connection will be terminated by
the proxy. If there is outstanding data to that peer undelivered,
that data will be discarded.
An origin server which receives a CONNECT request for itself &MAY;
respond with a 2xx status code to indicate that a connection is
established. However, most origin servers do not implement CONNECT.
Header fields are key value pairs that can be used to communicate data about
the message, its payload, the target resource, or about the connection
itself (i.e., control data). See &header-fields; for a general definition
of their syntax.
New header fields are registered using the procedures described in
.
The requirements for header field names are defined in
. Authors of specifications
defining new fields are advised to keep the name as short as practical, and
not to prefix them with "X-" if they are to be registered (either
immediately or in the future).
New header field values typically have their syntax defined using ABNF
(), using the extension defined in &abnf-extension;
as necessary, and are usually constrained to the range of ASCII characters.
Header fields needing a greater range of characters can use an encoding
such as the one defined in .
Because commas (",") are used as a generic delimiter between field-values,
they need to be treated with care if they are allowed in the field-value's
payload. Typically, components that might contain a comma are protected with
double-quotes using the quoted-string ABNF production (&field-components;).
For example, a textual date and a URI (either of which might contain a comma)
could be safely carried in field-values like these:
Example-URI-Field: "http://example.com/a.html,foo",
"http://without-a-comma.example.com/"
Example-Date-Field: "Sat, 04 May 1996", "Wed, 14 Sep 2005"
Note that double quote delimiters almost always are used with the
quoted-string production; using a different syntax inside double quotes
will likely cause unnecessary confusion.
Many header fields use a format including (case-insensitively) named
parameters (for instance, Content-Type, defined in
&header-content-type;). Allowing both unquoted (token) and quoted
(quoted-string) syntax for the parameter value enables recipients to use
existing parser components. When allowing both forms, the meaning of a
parameter value ought to be independent of the syntax used for it (for an
example, see the notes on parameter handling for media types in
&media-types;).
Authors of specifications defining new header fields are advised to consider
documenting:
Whether the field is a single value, or whether it can be a list
(delimited by commas; see &header-fields;).If it does not use the list syntax, document how to treat messages
where the header field occurs multiple times (a sensible default would
be to ignore the header field, but this might not always be the right
choice).Note that intermediaries and software libraries might combine
multiple header field instances into a single one, despite the header
field not allowing this. A robust format enables recipients to discover
these situations (good example: "Content-Type", as the comma can only
appear inside quoted strings; bad example: "Location", as a comma can
occur inside a URI).Under what conditions the header field can be used; e.g., only in
responses or requests, in all messages, only on responses to a particular
request method.Whether it is appropriate to list the field-name in the
Connection header field (i.e., if the header field is to
be hop-by-hop, see &header-connection;).Under what conditions intermediaries are allowed to modify the header
field's value, insert or delete it.How the header field might interact with caching (see
).Whether the header field is useful or allowable in trailers (see
&chunked-encoding;).Whether the header field ought to be preserved across redirects.
The request header fields allow the client to pass additional
information about the request, and about the client itself, to the
server. These fields act as request modifiers, with semantics
equivalent to the parameters on a programming language method
invocation.
Header Field NameDefined in...Accept&header-accept;Accept-Charset&header-accept-charset;Accept-Encoding&header-accept-encoding;Accept-Language&header-accept-language;Authorization&header-authorization;ExpectFromHost&header-host;If-Match&header-if-match;If-Modified-Since&header-if-modified-since;If-None-Match&header-if-none-match;If-Range&header-if-range;If-Unmodified-Since&header-if-unmodified-since;Max-ForwardsProxy-Authorization&header-proxy-authorization;Range&header-range;RefererTE&header-te;User-Agent
The response header fields allow the server to pass additional
information about the response which cannot be placed in the status-line.
These header fields give information about the server and about
further access to the target resource (&effective-request-uri;).
Header Field NameDefined in...Accept-Ranges&header-accept-ranges;Age&header-age;AllowDateETag&header-etag;LocationProxy-Authenticate&header-proxy-authenticate;Retry-AfterServerVary&header-vary;WWW-Authenticate&header-www-authenticate;
The status-code element is a 3-digit integer result code of the attempt to
understand and satisfy the request.
HTTP status codes are extensible. HTTP applications are not required
to understand the meaning of all registered status codes, though such
understanding is obviously desirable. However, applications &MUST;
understand the class of any status code, as indicated by the first
digit, and treat any unrecognized response as being equivalent to the
x00 status code of that class, with the exception that an
unrecognized response &MUST-NOT; be cached. For example, if an
unrecognized status code of 431 is received by the client, it can
safely assume that there was something wrong with its request and
treat the response as if it had received a 400 status code. In such
cases, user agents &SHOULD; present to the user the representation enclosed
with the response, since that representation is likely to include human-readable
information which will explain the unusual status.
The first digit of the status-code defines the class of response. The
last two digits do not have any categorization role. There are 5
values for the first digit:
1xx (Informational): Request received, continuing process
2xx (Successful): The action was successfully received,
understood, and accepted
3xx (Redirection): Further action needs to be taken in order to
complete the request
4xx (Client Error): The request contains bad syntax or cannot
be fulfilled
5xx (Server Error): The server failed to fulfill an apparently
valid request
For most status codes the response can carry a payload, in which case a
Content-Type header field indicates the payload's media type
(&header-content-type;).
The status codes listed below are defined in this specification,
&p4-status-codes;, &p5-status-codes;, and &p7-status-codes;.
The reason phrases listed here are only recommendations — they can be
replaced by local equivalents without affecting the protocol.
status-codereason-phraseDefined in...100Continue101Switching Protocols200OK201Created202Accepted203Non-Authoritative Information204No Content205Reset Content206Partial Content&status-206;300Multiple Choices301Moved Permanently302Found303See Other304Not Modified&status-304;305Use Proxy307Temporary Redirect400Bad Request401Unauthorized&status-401;402Payment Required403Forbidden404Not Found405Method Not Allowed406Not Acceptable407Proxy Authentication Required&status-407;408Request Time-out409Conflict410Gone411Length Required412Precondition Failed&status-412;413Request Representation Too Large414URI Too Long415Unsupported Media Type416Requested range not satisfiable&status-416;417Expectation Failed426Upgrade Required500Internal Server Error501Not Implemented502Bad Gateway503Service Unavailable504Gateway Time-out505HTTP Version not supported
Note that this list is not exhaustive — it does not include
extension status codes defined in other specifications.
The HTTP Status Code Registry defines the name space for the status-code
token in the status-line of an HTTP response.
Values to be added to this name space require IETF Review
(see ).
The registry itself is maintained at .
When it is necessary to express new semantics for a HTTP response that
aren't specific to a single application or media type, and currently defined
status codes are inadequate, a new status code can be registered.
HTTP status codes are generic; that is, they are potentially applicable to
any resource, not just one particular media type, "type" of resource, or
application. As such, it is preferred that new HTTP status codes be
registered in a document that isn't specific to a single application, so
that this is clear.
Definitions of new HTTP status codes typically explain the request
conditions that produce a response containing the status code (e.g.,
combinations of request header fields and/or method(s)), along with any
interactions with response header fields (e.g., those that are required,
those that modify the semantics of the response).
New HTTP status codes are required to fall under one of the categories
defined in . To allow existing parsers to
properly handle them, new status codes cannot disallow a response body,
although they can mandate a zero-length response body. They can require the
presence of one or more particular HTTP response header field(s).
Likewise, their definitions can specify that caches are allowed to use
heuristics to determine their freshness (see &caching;; by default, it is
not allowed), and can define how to determine the resource which they
carry a representation for (see ; by default,
it is anonymous).
This class of status code indicates a provisional response,
consisting only of the status-line and optional header fields, and is
terminated by an empty line. There are no required header fields for this
class of status code. Since HTTP/1.0 did not define any 1xx status
codes, servers &MUST-NOT; send a 1xx response to an HTTP/1.0 client
except under experimental conditions.
A client &MUST; be prepared to accept one or more 1xx status responses
prior to a regular response, even if the client does not expect a 100
(Continue) status message. Unexpected 1xx status responses &MAY; be
ignored by a user agent.
Proxies &MUST; forward 1xx responses, unless the connection between the
proxy and its client has been closed, or unless the proxy itself
requested the generation of the 1xx response. (For example, if a
proxy adds an "Expect: 100-continue" field when it forwards a request,
then it need not forward the corresponding 100 (Continue)
response(s).)
The client &SHOULD; continue with its request. This interim response is
used to inform the client that the initial part of the request has
been received and has not yet been rejected by the server. The client
&SHOULD; continue by sending the remainder of the request or, if the
request has already been completed, ignore this response. The server
&MUST; send a final response after the request has been completed. See
&use100; for detailed discussion of the use and handling of this
status code.
The server understands and is willing to comply with the client's request,
via the Upgrade message header field (&header-upgrade;), for
a change in the application protocol being used on this connection. The
server will switch protocols to those defined by the response's Upgrade
header field immediately after the empty line which terminates the 101
response.
The protocol &SHOULD; be switched only when it is advantageous to do
so. For example, switching to a newer version of HTTP is advantageous
over older versions, and switching to a real-time, synchronous
protocol might be advantageous when delivering resources that use
such features.
This class of status code indicates that the client's request was
successfully received, understood, and accepted.
The request has succeeded. The payload returned with the response
is dependent on the method used in the request, for example:
a representation of the target resource is sent in the response;
the same representation as GET, except without the message body;
a representation describing or containing the result of the action;
a representation containing the request message as received by the
end server.
Caches &MAY; use a heuristic (see &p6-heuristic;) to determine
freshness for 200 responses.
The request has been fulfilled and has resulted in one or more new resources
being created.
Newly created resources are typically linked to from the response payload,
with the most relevant URI also being carried in the Location
header field. If the newly created resource's URI is the same as the
Effective Request URI, this information can be omitted (e.g., in the case of
a response to a PUT request).
The origin server &MUST; create the resource(s) before returning the 201 status
code. If the action cannot be carried out immediately, the server &SHOULD;
respond with 202 (Accepted) response instead.
A 201 response &MAY; contain an ETag response header field
indicating the current value of the entity-tag for the representation of the
resource identified by the Location header field or, in case
the Location header field was omitted, by the Effective Request URI (see
&header-etag;).
The request has been accepted for processing, but the processing has
not been completed. The request might or might not eventually be
acted upon, as it might be disallowed when processing actually takes
place. There is no facility for re-sending a status code from an
asynchronous operation such as this.
The 202 response is intentionally non-committal. Its purpose is to
allow a server to accept a request for some other process (perhaps a
batch-oriented process that is only run once per day) without
requiring that the user agent's connection to the server persist
until the process is completed. The representation returned with this
response &SHOULD; include an indication of the request's current status
and either a pointer to a status monitor or some estimate of when the
user can expect the request to be fulfilled.
The representation in the response has been transformed or otherwise
modified by a transforming proxy (&intermediaries;). Note that the
behavior of transforming intermediaries is controlled by the no-transform
Cache-Control directive (&header-cache-control;).
This status code is only appropriate when the response status code would
have been 200 (OK) otherwise. When the status code before
transformation would have been different, the 214 Transformation Applied
warn-code (&header-warning;) is appropriate.
Caches &MAY; use a heuristic (see &p6-heuristic;) to determine
freshness for 203 responses.
The 204 (No Content) status code indicates that the server has
successfully fulfilled the request and that there is no additional
content to return in the response payload body. Metadata in the
response header fields refer to the target resource and its current
representation after the requested action.
For example, if a 204 status code is received in response to a PUT
request and the response contains an ETag header field, then
the PUT was successful and the ETag field-value contains the entity-tag for
the new representation of that target resource.
The 204 response allows a server to indicate that the action has been
successfully applied to the target resource while implying that the
user agent &SHOULD-NOT; traverse away from its current "document view"
(if any). The server assumes that the user agent will provide some
indication of the success to its user, in accord with its own interface,
and apply any new or updated metadata in the response to the active
representation.
For example, a 204 status code is commonly used with document editing
interfaces corresponding to a "save" action, such that the document
being saved remains available to the user for editing. It is also
frequently used with interfaces that expect automated data transfers
to be prevalent, such as within distributed version control systems.
The 204 response &MUST-NOT; include a message body, and thus is always
terminated by the first empty line after the header fields.
The server has fulfilled the request and the user agent &SHOULD; reset
the document view which caused the request to be sent. This response
is primarily intended to allow input for actions to take place via
user input, followed by a clearing of the form in which the input is
given so that the user can easily initiate another input action.
The message body included with the response &MUST; be empty. Note that
receivers still need to parse the response according to the algorithm defined
in &message-body;.
This class of status code indicates that further action needs to be
taken by the user agent in order to fulfill the request. If the required
action involves a subsequent HTTP request, it &MAY; be carried out by the
user agent without interaction with the user if and only if the method used
in the second request is known to be "safe", as defined in
.
There are several types of redirects:
Redirects of the request to another URI, either temporarily or
permanently. The new URI is specified in the Location
header field. In this specification, the status codes 301
(Moved Permanently), 302 (Found), and
307 (Temporary Redirect) fall under this category.
Redirection to a new location that represents an indirect response to
the request, such as the result of a POST operation to be retrieved
with a subsequent GET request. This is status code 303 (See
Other).
Redirection offering a choice of matching resources for use by
agent-driven content negotiation (&agent-driven-negotiation;). This
is status code 300 (Multiple Choices).
Other kinds of redirection, such as to a cached result (status code 304
(Not Modified), see &status-304;).
&Note; In HTTP/1.0, only the status codes 301 (Moved Permanently)
and 302 (Found) were defined for the first type of redirect,
and the second type did not exist at all ().
However it turned out that web forms using POST expected redirects to change
the operation for the subsequent request to retrieval (GET). To address this
use case, HTTP/1.1 introduced the second type of redirect with the status
code 303 (See Other) ().
As user agents did not change their behavior to maintain backwards
compatibility, the first revision of HTTP/1.1 added yet another status code,
307 (Temporary Redirect), for which the backwards
compatibility problems did not apply ().
Over 10 years later, most user agents still do method rewriting for status codes
301 and 302, therefore this specification makes that behavior
conformant in case the original request was POST.
A Location header field on a 3xx response indicates that a
client &MAY; automatically redirect to the URI provided; see
.
Note that for methods not known to be "safe", as defined in ,
automatic redirection needs to done with care, since the redirect might
change the conditions under which the request was issued.
Clients &SHOULD; detect and intervene in cyclical redirections (i.e.,
"infinite" redirection loops).
&Note; An earlier version of this specification recommended a
maximum of five redirections ().
Content developers need to be aware that some clients might
implement such a fixed limitation.
The target resource has more than one
representation, each with its own specific location, and agent-driven
negotiation information (&content-negotiation;) is being provided so that
the user (or user agent) can select a preferred representation by
redirecting its request to that location.
Unless it was a HEAD request, the response &SHOULD; include a representation
containing a list of representation metadata and location(s) from
which the user or user agent can choose the one most appropriate. Depending
upon the format and the capabilities of
the user agent, selection of the most appropriate choice &MAY; be
performed automatically. However, this specification does not define
any standard for such automatic selection.
If the server has a preferred choice of representation, it &SHOULD;
include the specific URI for that representation in the Location
field; user agents &MAY; use the Location field value for automatic
redirection.
Caches &MAY; use a heuristic (see &p6-heuristic;) to determine
freshness for 300 responses.
The target resource has been assigned a new permanent URI and any
future references to this resource &SHOULD; use one of the returned
URIs. Clients with link editing capabilities ought to automatically
re-link references to the effective request URI to one or more of the new
references returned by the server, where possible.
Caches &MAY; use a heuristic (see &p6-heuristic;) to determine
freshness for 301 responses.
The new permanent URI &SHOULD; be given by the Location field
in the response. A response payload can contain a short hypertext note with a
hyperlink to the new URI(s).
&Note; For historic reasons, user agents &MAY; change the
request method from POST to GET for the subsequent request. If this
behavior is undesired, status code 307 (Temporary Redirect)
can be used instead.
The target resource resides temporarily under a different URI.
Since the redirection might be altered on occasion, the client &SHOULD;
continue to use the effective request URI for future requests.
The temporary URI &SHOULD; be given by the Location field in
the response. A response payload can contain a short hypertext note with a
hyperlink to the new URI(s).
&Note; For historic reasons, user agents &MAY; change the
request method from POST to GET for the subsequent request. If this
behavior is undesired, status code 307 (Temporary Redirect)
can be used instead.
The 303 status code indicates that the server is redirecting the
user agent to a different resource, as indicated by a URI in the
Location header field, that is intended to provide an indirect
response to the original request. In order to satisfy the original
request, a user agent &SHOULD; perform a retrieval request using the
Location URI (a GET or HEAD request if using HTTP), which
can itself be redirected further, and present the eventual result as an
answer to the original request.
Note that the new URI in the Location header field is not considered
equivalent to the effective request URI.
This status code is generally applicable to any HTTP method. It is
primarily used to allow the output of a POST action to redirect
the user agent to a selected resource, since doing so provides the
information corresponding to the POST response in a form that
can be separately identified, bookmarked, and cached independent
of the original request.
A 303 response to a GET request indicates that the requested
resource does not have a representation of its own that can be
transferred by the server over HTTP. The Location URI
indicates a resource that is descriptive of the target resource, such that
the follow-on representation might be useful to recipients without
implying that it adequately represents the target resource.
Note that answers to the questions of what can be represented, what
representations are adequate, and what might be a useful description
are outside the scope of HTTP and thus entirely determined by the
URI owner(s).
Except for responses to a HEAD request, the representation of a 303
response &SHOULD; contain a short hypertext note with a hyperlink
to the Location URI.
The 305 status code was defined in a previous version of this specification
(see ), and is now deprecated.
The 306 status code was used in a previous version of the
specification, is no longer used, and the code is reserved.
The target resource resides temporarily under a different URI.
Since the redirection can change over time, the client &SHOULD;
continue to use the effective request URI for future requests.
The temporary URI &SHOULD; be given by the Location field in
the response. A response payload can contain a short hypertext note with a
hyperlink to the new URI(s).
&Note; This status code is similar to 302 (Found), except
that it does not allow rewriting the request method from POST to GET. This
specification defines no equivalent counterpart for 301 (Moved
Permanently) (,
however, defines the status code 308 (Permanent Redirect) for this purpose).
The 4xx class of status code is intended for cases in which the
client seems to have erred. Except when responding to a HEAD request,
the server &SHOULD; include a representation containing an explanation of the
error situation, and whether it is a temporary or permanent
condition. These status codes are applicable to any request method.
User agents &SHOULD; display any included representation to the user.
The server cannot or will not process the request, due to a client error (e.g.,
malformed syntax).
This code is reserved for future use.
The server understood the request, but refuses to authorize it. Providing
different user authentication credentials might be successful, but any
credentials that were provided in the request are insufficient. The request
&SHOULD-NOT; be repeated with the same credentials.
If the request method was not HEAD and the server wishes to make
public why the request has not been fulfilled, it &SHOULD; describe the
reason for the refusal in the representation. If the server does not wish to
make this information available to the client, the status code 404
(Not Found) &MAY; be used instead.
The server has not found anything matching the effective request URI. No
indication is given of whether the condition is temporary or
permanent. The 410 (Gone) status code &SHOULD; be used if the server
knows, through some internally configurable mechanism, that an old
resource is permanently unavailable and has no forwarding address.
This status code is commonly used when the server does not wish to
reveal exactly why the request has been refused, or when no other
response is applicable.
The method specified in the request-line is not allowed for the target
resource. The response &MUST; include an Allow header field
containing a list of valid methods for the requested resource.
The resource identified by the request is only capable of generating
response representations which have content characteristics not acceptable
according to the Accept and Accept-* header fields sent in
the request.
Unless it was a HEAD request, the response &SHOULD; include a representation
containing a list of available representation characteristics and location(s)
from which the user or user agent can choose the one most
appropriate. Depending upon the format and the
capabilities of the user agent, selection of the most appropriate
choice &MAY; be performed automatically. However, this specification
does not define any standard for such automatic selection.
&Note; HTTP/1.1 servers are allowed to return responses which are
not acceptable according to the accept header fields sent in the
request. In some cases, this might even be preferable to sending a
406 response. User agents are encouraged to inspect the header fields of
an incoming response to determine if it is acceptable.
If the response could be unacceptable, a user agent &SHOULD;
temporarily stop receipt of more data and query the user for a
decision on further actions.
The client did not produce a request within the time that the server
was prepared to wait. The client &MAY; repeat the request without
modifications at any later time.
The request could not be completed due to a conflict with the current
state of the resource. This code is only allowed in situations where
it is expected that the user might be able to resolve the conflict
and resubmit the request. The response body &SHOULD; include enough
information for the user to recognize the source of the conflict.
Ideally, the response representation would include enough information for the
user or user agent to fix the problem; however, that might not be
possible and is not required.
Conflicts are most likely to occur in response to a PUT request. For
example, if versioning were being used and the representation being PUT
included changes to a resource which conflict with those made by an
earlier (third-party) request, the server might use the 409 response
to indicate that it can't complete the request. In this case, the
response representation would likely contain a list of the differences
between the two versions.
The target resource is no longer available at the server and no
forwarding address is known. This condition is expected to be
considered permanent. Clients with link editing capabilities &SHOULD;
delete references to the effective request URI after user approval. If the
server does not know, or has no facility to determine, whether or not
the condition is permanent, the status code 404 (Not Found)
&SHOULD; be used instead.
The 410 response is primarily intended to assist the task of web
maintenance by notifying the recipient that the resource is
intentionally unavailable and that the server owners desire that
remote links to that resource be removed. Such an event is common for
limited-time, promotional services and for resources belonging to
individuals no longer working at the server's site. It is not
necessary to mark all permanently unavailable resources as "gone" or
to keep the mark for any length of time — that is left to the
discretion of the server owner.
Caches &MAY; use a heuristic (see &p6-heuristic;) to determine freshness
for 410 responses.
The server refuses to accept the request without a defined
Content-Length. The client &MAY; repeat the request if it
adds a valid Content-Length header field containing the length of the
message body in the request message.
The server is refusing to process a request because the request
representation is larger than the server is willing or able to process. The
server &MAY; close the connection to prevent the client from continuing
the request.
If the condition is temporary, the server &SHOULD; include a
Retry-After header field to indicate that it is temporary and
after what time the client &MAY; try again.
The server is refusing to service the request because the effective request URI
is longer than the server is willing to interpret. This rare
condition is only likely to occur when a client has improperly
converted a POST request to a GET request with long query
information, when the client has descended into a URI "black hole" of
redirection (e.g., a redirected URI prefix that points to a suffix of
itself), or when the server is under attack by a client attempting to
exploit security holes present in some servers using fixed-length
buffers for reading or manipulating the request-target.
The server is refusing to service the request because the request
payload is in a format not supported by this request method on the
target resource.
The expectation given in an Expect header field (see
) could not be met by this server, or, if the
server is a proxy, the server has unambiguous evidence that the request
could not be met by the next-hop server.
The request can not be completed without a prior protocol upgrade. This
response &MUST; include an Upgrade header field
(&header-upgrade;) specifying the required protocols.
Example:
HTTP/1.1 426 Upgrade Required
Upgrade: HTTP/3.0
Connection: Upgrade
Content-Length:
Content-Type: text/plain
This service requires use of the HTTP/3.0 protocol.
The server &SHOULD; include a message body in the 426 response which
indicates in human readable form the reason for the error and describes any
alternative courses which might be available to the user.
Response status codes beginning with the digit "5" indicate cases in
which the server is aware that it has erred or is incapable of
performing the request. Except when responding to a HEAD request, the
server &SHOULD; include a representation containing an explanation of the
error situation, and whether it is a temporary or permanent
condition. User agents &SHOULD; display any included representation to the
user. These response codes are applicable to any request method.
The server encountered an unexpected condition which prevented it
from fulfilling the request.
The server does not support the functionality required to fulfill the
request. This is the appropriate response when the server does not
recognize the request method and is not capable of supporting it for
any resource.
The server, while acting as a gateway or proxy, received an invalid
response from the upstream server it accessed in attempting to
fulfill the request.
The server is currently unable to handle the request due to a
temporary overloading or maintenance of the server.
The implication is that this is a temporary condition which will be
alleviated after some delay. If known, the length of the delay &MAY; be
indicated in a Retry-After header field
(). If no Retry-After is given, the
client &SHOULD; handle the response as it would for a 500 (Internal
Server Error) response.
&Note; The existence of the 503 status code does not imply that a
server has to use it when becoming overloaded. Some servers might wish
to simply refuse the connection.
The server, while acting as a gateway or proxy, did not receive a
timely response from the upstream server specified by the URI (e.g.,
HTTP, FTP, LDAP) or some other auxiliary server (e.g., DNS) it needed
to access in attempting to complete the request.
Note to implementers: some deployed proxies are known to
return 400 (Bad Request) or 500 (Internal Server
Error) when DNS lookups time out.
The server does not support, or refuses to support, the protocol
version that was used in the request message. The server is
indicating that it is unable or unwilling to complete the request
using the same major version as the client, as described in &http-version;,
other than with this error message. The response &SHOULD; contain
a representation describing why that version is not supported and what other
protocols are supported by that server.
HTTP applications have historically allowed three different formats
for date/time stamps. However, the preferred format is a fixed-length subset
of that defined by :
Sun, 06 Nov 1994 08:49:37 GMT ; RFC 1123
The other formats are described here only for compatibility with obsolete
implementations.
Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format
HTTP/1.1 clients and servers that parse a date value &MUST; accept
all three formats (for compatibility with HTTP/1.0), though they &MUST;
only generate the RFC 1123 format for representing HTTP-date values
in header fields.
All HTTP date/time stamps &MUST; be represented in Greenwich Mean Time
(GMT), without exception. For the purposes of HTTP, GMT is exactly
equal to UTC (Coordinated Universal Time). This is indicated in the
first two formats by the inclusion of "GMT" as the three-letter
abbreviation for time zone, and &MUST; be assumed when reading the
asctime format. HTTP-date is case sensitive and &MUST-NOT; include
additional whitespace beyond that specifically included as SP in the
grammar.
HTTP-date = rfc1123-date / obs-date
Preferred format:
rfc1123-date = day-name "," SP date1 SPtime-of-daySPGMT
; fixed length subset of the format defined in
; day-name = "Mon" ; "Mon", case-sensitive
/ "Tue" ; "Tue", case-sensitive
/ "Wed" ; "Wed", case-sensitive
/ "Thu" ; "Thu", case-sensitive
/ "Fri" ; "Fri", case-sensitive
/ "Sat" ; "Sat", case-sensitive
/ "Sun" ; "Sun", case-sensitive
date1 = daySPmonthSPyear
; e.g., 02 Jun 1982
day = 2DIGITmonth = "Jan" ; "Jan", case-sensitive
/ "Feb" ; "Feb", case-sensitive
/ "Mar" ; "Mar", case-sensitive
/ "Apr" ; "Apr", case-sensitive
/ "May" ; "May", case-sensitive
/ "Jun" ; "Jun", case-sensitive
/ "Jul" ; "Jul", case-sensitive
/ "Aug" ; "Aug", case-sensitive
/ "Sep" ; "Sep", case-sensitive
/ "Oct" ; "Oct", case-sensitive
/ "Nov" ; "Nov", case-sensitive
/ "Dec" ; "Dec", case-sensitive
year = 4DIGITGMT = "GMT" ; "GMT", case-sensitive
time-of-day = hour ":" minute ":" second
; 00:00:00 - 23:59:59
hour = 2DIGITminute = 2DIGITsecond = 2DIGIT
The semantics of day-name, day,
month, year, and time-of-day are the
same as those defined for the RFC 5322 constructs
with the corresponding name ().
Obsolete formats:
obs-date = rfc850-date / asctime-daterfc850-date = day-name-l "," SPdate2SPtime-of-daySPGMTdate2 = day "-" month "-" 2DIGIT
; day-month-year (e.g., 02-Jun-82)
day-name-l = "Monday" ; "Monday", case-sensitive
/ "Tuesday" ; "Tuesday", case-sensitive
/ "Wednesday" ; "Wednesday", case-sensitive
/ "Thursday" ; "Thursday", case-sensitive
/ "Friday" ; "Friday", case-sensitive
/ "Saturday" ; "Saturday", case-sensitive
/ "Sunday" ; "Sunday", case-sensitive
asctime-date = day-nameSPdate3SPtime-of-daySPyeardate3 = monthSP ( 2DIGIT / ( SP 1DIGIT ))
; month day (e.g., Jun 2)
&Note; Recipients of date values are encouraged to be robust in
accepting date values that might have been sent by non-HTTP
applications, as is sometimes the case when retrieving or posting
messages via proxies/gateways to SMTP or NNTP.
&Note; HTTP requirements for the date/time stamp format apply only
to their usage within the protocol stream. Clients and servers are
not required to use these formats for user presentation, request
logging, etc.
Product tokens are used to allow communicating applications to
identify themselves by software name and version. Most fields using
product tokens also allow sub-products which form a significant part
of the application to be listed, separated by whitespace. By
convention, the products are listed in order of their significance
for identifying the application.
product = token ["/" product-version]
product-version = token
Examples:
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
Server: Apache/0.8.4
Product tokens &SHOULD; be short and to the point. They &MUST-NOT; be
used for advertising or other non-essential information. Although any
token octet &MAY; appear in a product-version, this token &SHOULD;
only be used for a version identifier (i.e., successive versions of
the same product &SHOULD; only differ in the product-version portion of
the product value).
HTTP uses charset names to indicate the character encoding of a
textual representation.
A character encoding is identified by a case-insensitive token. The
complete set of tokens is defined by the IANA Character Set registry
().
charset = token
Although HTTP allows an arbitrary token to be used as a charset
value, any token that has a predefined value within the IANA
Character Set registry &MUST; represent the character encoding defined
by that registry. Applications &SHOULD; limit their use of character
encodings to those defined within the IANA registry.
HTTP uses charset in two contexts: within an Accept-Charset
request header field (in which the charset value is an unquoted token) and
as the value of a parameter in a Content-Type header field
(within a request or response), in which case the parameter value of the
charset parameter can be quoted.
Implementers need to be aware of IETF character set requirements .
Content coding values indicate an encoding transformation that has
been or can be applied to a representation. Content codings are primarily
used to allow a representation to be compressed or otherwise usefully
transformed without losing the identity of its underlying media type
and without loss of information. Frequently, the representation is stored in
coded form, transmitted directly, and only decoded by the recipient.
content-coding = token
All content-coding values are case-insensitive. HTTP/1.1 uses
content-coding values in the Accept-Encoding
() and Content-Encoding
() header fields. Although the value
describes the content-coding, what is more important is that it
indicates what decoding mechanism will be required to remove the
encoding.
compress
See &compress-coding;.
deflate
See &deflate-coding;.
gzip
See &gzip-coding;.
The HTTP Content Coding Registry defines the name space for the content
coding names.
Registrations &MUST; include the following fields:
NameDescriptionPointer to specification text
Names of content codings &MUST-NOT; overlap with names of transfer codings
(&transfer-codings;), unless the encoding transformation is identical (as
is the case for the compression codings defined in
&compression-codings;).
Values to be added to this name space require IETF Review
(see ), and &MUST;
conform to the purpose of content coding defined in this section.
The registry itself is maintained at
.
HTTP uses Internet Media Types in the
Content-Type ()
and Accept () header fields in
order to provide open and extensible data typing and type negotiation.
media-type = type "/" subtype *( OWS ";" OWSparameter )
type = tokensubtype = token
The type/subtype &MAY; be followed by parameters in the form of
attribute/value pairs.
parameter = attribute "=" valueattribute = tokenvalue = word
The type, subtype, and parameter attribute names are case-insensitive.
Parameter values might or might not be case-sensitive, depending on the
semantics of the parameter name. The presence or absence of a parameter might
be significant to the processing of a media-type, depending on its
definition within the media type registry.
A parameter value that matches the token production can be
transmitted as either a token or within a quoted-string. The quoted and
unquoted values are equivalent.
Note that some older HTTP applications do not recognize media type
parameters. When sending data to older HTTP applications,
implementations &SHOULD; only use media type parameters when they are
required by that type/subtype definition.
Media-type values are registered with the Internet Assigned Number
Authority (IANA). The media type registration process is
outlined in . Use of non-registered media types is
discouraged.
Internet media types are registered with a canonical form. A
representation transferred via HTTP messages &MUST; be in the
appropriate canonical form prior to its transmission except for
"text" types, as defined in the next paragraph.
When in canonical form, media subtypes of the "text" type use CRLF as
the text line break. HTTP relaxes this requirement and allows the
transport of text media with plain CR or LF alone representing a line
break when it is done consistently for an entire representation. HTTP
applications &MUST; accept CRLF, bare CR, and bare LF as indicating
a line break in text media received via HTTP. In
addition, if the text is in a character encoding that does not
use octets 13 and 10 for CR and LF respectively, as is the case for
some multi-byte character encodings, HTTP allows the use of whatever octet
sequences are defined by that character encoding to represent the
equivalent of CR and LF for line breaks. This flexibility regarding
line breaks applies only to text media in the payload body; a bare CR
or LF &MUST-NOT; be substituted for CRLF within any of the HTTP control
structures (such as header fields and multipart boundaries).
If a representation is encoded with a content-coding, the underlying
data &MUST; be in a form defined above prior to being encoded.
MIME provides for a number of "multipart" types — encapsulations of
one or more representations within a single message body. All multipart
types share a common syntax, as defined in ,
and &MUST; include a boundary parameter as part of the media type
value. The message body is itself a protocol element and &MUST;
therefore use only CRLF to represent line breaks between body-parts.
In general, HTTP treats a multipart message body no differently than
any other media type: strictly as payload. HTTP does not use the
multipart boundary as an indicator of message body length.
In all other respects, an HTTP user agent &SHOULD; follow the same or similar
behavior as a MIME user agent would upon receipt of a multipart type.
The MIME header fields within each body-part of a multipart message body
do not have any significance to HTTP beyond that defined by
their MIME semantics.
If an application receives an unrecognized multipart subtype, the
application &MUST; treat it as being equivalent to "multipart/mixed".
&Note; The "multipart/form-data" type has been specifically defined
for carrying form data suitable for processing via the POST
request method, as described in .
A language tag, as defined in , identifies a
natural language spoken, written, or otherwise conveyed by human beings for
communication of information to other human beings. Computer languages are
explicitly excluded. HTTP uses language tags within the
Accept-Language and Content-Language fields.
In summary, a language tag is composed of one or more parts: A primary
language subtag followed by a possibly empty series of subtags:
language-tag = <Language-Tag, defined in >
White space is not allowed within the tag and all tags are case-insensitive.
The name space of language subtags is administered by the IANA (see
).
Example tags include:
en, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN
See for further information.
HTTP messages &MAY; transfer a payload if not otherwise restricted by
the request method or response status code. The payload consists of
metadata, in the form of header fields, and data, in the form of the
sequence of octets in the message body after any transfer-coding has
been decoded.
A "payload" in HTTP is always a partial or complete
representation of some resource. We use separate terms for payload
and representation because some messages contain only the associated
representation's header fields (e.g., responses to HEAD) or only some
part(s) of the representation (e.g., the 206 (Partial Content)
status code).
HTTP header fields that specifically define the payload, rather than the
associated representation, are referred to as "payload header fields".
The following payload header fields are defined by HTTP/1.1:
Header Field NameDefined in...Content-Length&header-content-length;Content-Range&header-content-range;
A payload body is only present in a message when a message body is
present, as described in &message-body;. The payload body is obtained
from the message body by decoding any Transfer-Encoding that
might have been applied to ensure safe and proper transfer of the message.
A "representation" is information in a format that can be readily
communicated from one party to another. A resource representation
is information that reflects the state of that resource, as observed
at some point in the past (e.g., in a response to GET) or to be
desired at some point in the future (e.g., in a PUT request).
Most, but not all, representations transferred via HTTP are intended
to be a representation of the target resource (the resource identified
by the effective request URI). The precise semantics of a representation
are determined by the type of message (request or response), the request
method, the response status code, and the representation metadata. For
example, the above semantic is true for the representation in any
200 (OK) response to GET and for the representation in any PUT request.
A 200 response to PUT, in contrast, contains either a representation
that describes the successful action or a representation of the target
resource, with the latter indicated by a Content-Location
header field with the same value as the effective request URI. Likewise,
response messages with an error status code usually contain a representation
that describes the error and what next steps are suggested for resolving it.
Request and Response messages &MAY; transfer a representation if not otherwise
restricted by the request method or response status code. A representation
consists of metadata (representation header fields) and data (representation
body). When a complete or partial representation is enclosed in an HTTP message,
it is referred to as the payload of the message.
A representation body is only present in a message when a message body is
present, as described in &message-body;. The representation body is obtained
from the message body by decoding any Transfer-Encoding that
might have been applied to ensure safe and proper transfer of the message.
It is sometimes necessary to determine an identifier for the resource
associated with a representation.
An HTTP request representation, when present, is always associated with an
anonymous (i.e., unidentified) resource.
In the common case, an HTTP response is a representation of the target
resource (see &effective-request-uri;). However, this is not always the
case. To determine the URI of the resource a response is associated with,
the following rules are used (with the first applicable one being selected):
If the response status code is 200 (OK) or 203
(Non-Authoritative Information) and the request method was GET,
the response payload is a representation of the target resource.If the response status code is 204 (No Content),
206 (Partial Content), or 304 (Not Modified)
and the request method was GET or HEAD, the response payload is a partial
representation of the target resource.If the response has a Content-Location header field, and
that URI is the same as the effective request URI, the response payload is a
representation of the target resource.If the response has a Content-Location header field, and
that URI is not the same as the effective request URI, then the response
asserts that its payload is a representation of the resource identified by
the Content-Location URI. However, such an assertion cannot be trusted unless
it can be verified by other means (not defined by HTTP).Otherwise, the response is a representation of an anonymous (i.e.,
unidentified) resource.
The comparison function is going to have to be defined somewhere,
because we already need to compare URIs for things like cache invalidation.
Representation header fields define metadata about the representation data
enclosed in the message body or, if no message body is present, about
the representation that would have been transferred in a 200 (OK)
response to a simultaneous GET request with the same effective request URI.
The following header fields are defined as representation metadata:
Header Field NameDefined in...Content-EncodingContent-LanguageContent-LocationContent-TypeExpires&header-expires;
We use the term "selected representation" to refer to the
the current representation of a target resource that would have been
selected in a successful response if the same request had used the
method GET and excluded any conditional request header fields.
Additional header fields define metadata about the selected
representation, which might differ from the representation included
in the message for responses to some state-changing methods.
The following header fields are defined as selected representation
metadata:
Header Field NameDefined in...ETag&header-etag;Last-Modified&header-last-modified;
The representation body associated with an HTTP message is
either provided as the payload body of the message or
referred to by the message semantics and the effective request
URI. The representation data is in a format and encoding defined by
the representation metadata header fields.
The data type of the representation data is determined via the header fields
Content-Type and Content-Encoding.
These define a two-layer, ordered encoding model:
representation-data := Content-Encoding( Content-Type( bits ) )
Content-Type specifies the media type of the underlying data,
which defines both the data format and how that data &SHOULD; be processed
by the recipient (within the scope of the request method semantics).
Any HTTP/1.1 message containing a payload body &SHOULD; include a
Content-Type header field defining the media type of the associated
representation unless that metadata is unknown to the sender.
If the Content-Type header field is not present, it indicates that
the sender does not know the media type of the representation;
recipients &MAY; either assume that the media type is
"application/octet-stream" ()
or examine the content to determine its type.
In practice, resource owners do not always properly configure their origin
server to provide the correct Content-Type for a given representation,
with the result that some clients will examine a response body's content
and override the specified type.
Clients that do so risk drawing incorrect conclusions, which might expose
additional security risks (e.g., "privilege escalation"). Furthermore,
it is impossible to determine the sender's intent by examining the data
format: many data formats match multiple media types that differ only in
processing semantics. Implementers are encouraged to provide a means of
disabling such "content sniffing" when it is used.
Content-Encoding is used to indicate any additional content
codings applied to the data, usually for the purpose of data
compression, that are a property of the representation. If
Content-Encoding is not present, then there is no additional
encoding beyond that defined by the Content-Type header field.
HTTP responses include a representation which contains information for
interpretation, whether by a human user or for further processing.
Often, the server has different ways of representing the
same information; for example, in different formats, languages,
or using different character encodings.
HTTP clients and their users might have different or variable
capabilities, characteristics or preferences which would influence
which representation, among those available from the server,
would be best for the server to deliver. For this reason, HTTP
provides mechanisms for "content negotiation" — a process of
allowing selection of a representation of a given resource,
when more than one is available.
This specification defines two patterns of content negotiation;
"server-driven", where the server selects the representation based
upon the client's stated preferences, and "agent-driven" negotiation,
where the server provides a list of representations for the client to
choose from, based upon their metadata. In addition, there are
other patterns: some applications use an "active content" pattern,
where the server returns active content which runs on the client
and, based on client available parameters, selects additional
resources to invoke. "Transparent Content Negotiation" ()
has also been proposed.
These patterns are all widely used, and have trade-offs in applicability
and practicality. In particular, when the number of preferences or
capabilities to be expressed by a client are large (such as when many
different formats are supported by a user-agent), server-driven
negotiation becomes unwieldy, and might not be appropriate. Conversely,
when the number of representations to choose from is very large,
agent-driven negotiation might not be appropriate.
Note that in all cases, the supplier of representations has the
responsibility for determining which representations might be
considered to be the "same information".
If the selection of the best representation for a response is made by
an algorithm located at the server, it is called server-driven
negotiation. Selection is based on the available representations of
the response (the dimensions over which it can vary; e.g., language,
content-coding, etc.) and the contents of particular header fields in
the request message or on other information pertaining to the request
(such as the network address of the client).
Server-driven negotiation is advantageous when the algorithm for
selecting from among the available representations is difficult to
describe to the user agent, or when the server desires to send its
"best guess" to the client along with the first response (hoping to
avoid the round-trip delay of a subsequent request if the "best
guess" is good enough for the user). In order to improve the server's
guess, the user agent &MAY; include request header fields (Accept,
Accept-Language, Accept-Encoding, etc.) which describe its
preferences for such a response.
Server-driven negotiation has disadvantages:
It is impossible for the server to accurately determine what
might be "best" for any given user, since that would require
complete knowledge of both the capabilities of the user agent
and the intended use for the response (e.g., does the user want
to view it on screen or print it on paper?).
Having the user agent describe its capabilities in every
request can be both very inefficient (given that only a small
percentage of responses have multiple representations) and a
potential violation of the user's privacy.
It complicates the implementation of an origin server and the
algorithms for generating responses to a request.
It might limit a public cache's ability to use the same response
for multiple user's requests.
Server-driven negotiation allows the user agent to specify its preferences,
but it cannot expect responses to always honor them. For example, the origin
server might not implement server-driven negotiation, or it might decide that
sending a response that doesn't conform to them is better than sending a 406
(Not Acceptable) response.
Many of the mechanisms for expressing preferences use quality values to
declare relative preference. See &qvalue; for more information.
HTTP/1.1 includes the following header fields for enabling
server-driven negotiation through description of user agent
capabilities and user preferences: Accept
(), Accept-Charset
(), Accept-Encoding
(), Accept-Language
(), and User-Agent
(&header-user-agent;).
However, an origin server is not limited to these dimensions and &MAY; vary
the response based on any aspect of the request, including aspects
of the connection (e.g., IP address) or information within extension
header fields not defined by this specification.
&Note; In practice, User-Agent based negotiation is fragile,
because new clients might not be recognized.
The Vary header field (&header-vary;) can be used to express
the parameters the server uses to select a representation that is subject to
server-driven negotiation.
With agent-driven negotiation, selection of the best representation
for a response is performed by the user agent after receiving an
initial response from the origin server. Selection is based on a list
of the available representations of the response included within the
header fields or body of the initial response, with each
representation identified by its own URI. Selection from among the
representations can be performed automatically (if the user agent is
capable of doing so) or manually by the user selecting from a
generated (possibly hypertext) menu.
Agent-driven negotiation is advantageous when the response would vary
over commonly-used dimensions (such as type, language, or encoding),
when the origin server is unable to determine a user agent's
capabilities from examining the request, and generally when public
caches are used to distribute server load and reduce network usage.
Agent-driven negotiation suffers from the disadvantage of needing a
second request to obtain the best alternate representation. This
second request is only efficient when caching is used. In addition,
this specification does not define any mechanism for supporting
automatic selection, though it also does not prevent any such
mechanism from being developed as an extension and used within
HTTP/1.1.
This specification defines the 300 (Multiple Choices) and
406 (Not Acceptable) status codes for enabling agent-driven
negotiation when the server is unwilling or unable to provide a varying
response using server-driven negotiation.
This section defines the syntax and semantics of HTTP/1.1 header fields
related to request and response semantics and to the payload of messages.
The "Accept" header field can be used by user agents to specify
response media types that are acceptable. Accept header fields can be used to
indicate that the request is specifically limited to a small set of desired
types, as in the case of a request for an in-line image.
Accept = #( media-range [ accept-params ] )
media-range = ( "*/*"
/ ( type "/" "*" )
/ ( type "/" subtype )
) *( OWS ";" OWSparameter )
accept-params = OWS ";" OWS "q=" qvalue *( accept-ext )
accept-ext = OWS ";" OWStoken [ "=" word ]
The asterisk "*" character is used to group media types into ranges,
with "*/*" indicating all media types and "type/*" indicating all
subtypes of that type. The media-range &MAY; include media type
parameters that are applicable to that range.
Each media-range &MAY; be followed by one or more accept-params,
beginning with the "q" parameter for indicating a relative quality
factor. The first "q" parameter (if any) separates the media-range
parameter(s) from the accept-params. Quality factors allow the user
or user agent to indicate the relative degree of preference for that
media-range, using the qvalue scale from 0 to 1 (&qvalue;). The
default value is q=1.
&Note; Use of the "q" parameter name to separate media type
parameters from Accept extension parameters is due to historical
practice. Although this prevents any media type parameter named
"q" from being used with a media range, such an event is believed
to be unlikely given the lack of any "q" parameters in the IANA
media type registry and the rare usage of any media type
parameters in Accept. Future media types are discouraged from
registering any parameter named "q".
The example
Accept: audio/*; q=0.2, audio/basic
&SHOULD; be interpreted as "I prefer audio/basic, but send me any audio
type if it is the best available after an 80% mark-down in quality".
A request without any Accept header field implies that the user agent
will accept any media type in response.
If an Accept header field is present in a request and none of the
available representations for the response have a media type that is
listed as acceptable, the origin server &MAY; either honor the Accept
header field by sending a 406 (Not Acceptable) response
or disregard the Accept header field by treating the response as if
it is not subject to content negotiation.
A more elaborate example is
Accept: text/plain; q=0.5, text/html,
text/x-dvi; q=0.8, text/x-c
Verbally, this would be interpreted as "text/html and text/x-c are
the preferred media types, but if they do not exist, then send the
text/x-dvi representation, and if that does not exist, send the text/plain
representation".
Media ranges can be overridden by more specific media ranges or
specific media types. If more than one media range applies to a given
type, the most specific reference has precedence. For example,
Accept: text/*, text/plain, text/plain;format=flowed, */*
have the following precedence:
text/plain;format=flowedtext/plaintext/**/*
The media type quality factor associated with a given type is
determined by finding the media range with the highest precedence
which matches that type. For example,
Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
text/html;level=2;q=0.4, */*;q=0.5
would cause the following values to be associated:
Media TypeQuality Valuetext/html;level=11text/html0.7text/plain0.3image/jpeg0.5text/html;level=20.4text/html;level=30.7
&Note; A user agent might be provided with a default set of quality
values for certain media ranges. However, unless the user agent is
a closed system which cannot interact with other rendering agents,
this default set ought to be configurable by the user.
The "Accept-Charset" header field can be used by user agents to
indicate what character encodings are acceptable in a response
payload. This field allows
clients capable of understanding more comprehensive or special-purpose
character encodings to signal that capability to a server which is capable of
representing documents in those character encodings.
Accept-Charset = 1#( ( charset / "*" )
[ OWS ";" OWS "q=" qvalue ] )
Character encoding values (a.k.a., charsets) are described in
. Each charset &MAY; be given an
associated quality value which represents the user's preference
for that charset. The default value is q=1. An example is
Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
The special value "*", if present in the Accept-Charset field,
matches every character encoding which is not mentioned elsewhere in the
Accept-Charset field. If no "*" is present in an Accept-Charset field, then
all character encodings not explicitly mentioned get a quality value of 0.
A request without any Accept-Charset header field implies that the user
agent will accept any character encoding in response.
If an Accept-Charset header field is present in a request and none of the
available representations for the response have a character encoding that
is listed as acceptable, the origin server &MAY; either honor the
Accept-Charset header field by sending a 406 (Not Acceptable) response or
disregard the Accept-Charset header field by treating the response as if
it is not subject to content negotiation.
The "Accept-Encoding" header field can be used by user agents to
indicate what response content-codings ()
are acceptable in the response. An "identity" token is used as a synonym
for "no encoding" in order to communicate when no encoding is preferred.
Accept-Encoding = #( codings [ OWS ";" OWS "q=" qvalue ] )
codings = content-coding / "identity" / "*"
Each codings value &MAY; be given an associated quality value which
represents the preference for that encoding. The default value is q=1.
For example,
Accept-Encoding: compress, gzip
Accept-Encoding:
Accept-Encoding: *
Accept-Encoding: compress;q=0.5, gzip;q=1.0
Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0
A server tests whether a content-coding for a given representation is
acceptable, according to an Accept-Encoding field, using these rules:
The special "*" symbol in an Accept-Encoding field matches any
available content-coding not explicitly listed in the header
field.If the representation has no content-coding, then it is acceptable
by default unless specifically excluded by the Accept-Encoding field
stating either "identity;q=0" or "*;q=0" without a more specific
entry for "identity".If the representation's content-coding is one of the content-codings
listed in the Accept-Encoding field, then it is acceptable unless
it is accompanied by a qvalue of 0. (As defined in &qvalue;, a
qvalue of 0 means "not acceptable".)If multiple content-codings are acceptable, then the acceptable
content-coding with the highest non-zero qvalue is preferred.
An Accept-Encoding header field with a combined field-value that is empty
implies that the user agent does not want any content-coding in response.
If an Accept-Encoding header field is present in a request and none of the
available representations for the response have a content-coding that
is listed as acceptable, the origin server &SHOULD; send a response
without any content-coding.
A request without an Accept-Encoding header field implies that the user
agent will accept any content-coding in response, but a representation
without content-coding is preferred for compatibility with the widest
variety of user agents.
&Note; Most HTTP/1.0 applications do not recognize or obey qvalues
associated with content-codings. This means that qvalues will not
work and are not permitted with x-gzip or x-compress.
The "Accept-Language" header field can be used by user agents to
indicate the set of natural languages that are preferred in the response.
Language tags are defined in .
Accept-Language =
1#( language-range [ OWS ";" OWS "q=" qvalue ] )
language-range =
<language-range, defined in >
Each language-range can be given an associated quality value which
represents an estimate of the user's preference for the languages
specified by that range. The quality value defaults to "q=1". For
example,
Accept-Language: da, en-gb;q=0.8, en;q=0.7
would mean: "I prefer Danish, but will accept British English and
other types of English".
(see also )
For matching, defines
several matching schemes. Implementations can offer the most appropriate
matching scheme for their requirements.
&Note; The "Basic Filtering" scheme () is identical to the matching scheme that was
previously defined in .
It might be contrary to the privacy expectations of the user to send
an Accept-Language header field with the complete linguistic preferences of
the user in every request. For a discussion of this issue, see
.
As intelligibility is highly dependent on the individual user, it is
recommended that client applications make the choice of linguistic
preference available to the user. If the choice is not made
available, then the Accept-Language header field &MUST-NOT; be given in
the request.
&Note; When making the choice of linguistic preference available to
the user, we remind implementers of the fact that users are not
familiar with the details of language matching as described above,
and ought to be provided appropriate guidance. As an example, users
might assume that on selecting "en-gb", they will be served any
kind of English document if British English is not available. A
user agent might suggest in such a case to add "en" to get the
best matching behavior.
The "Allow" header field lists the set of methods advertised as
supported by the target resource. The purpose of this field is strictly to
inform the recipient of valid request methods associated with the resource.
Allow = #method
Example of use:
Allow: GET, HEAD, PUT
The actual set of allowed methods is defined by the origin server at the
time of each request.
A proxy &MUST-NOT; modify the Allow header field — it does not need to
understand all the methods specified in order to handle them according to
the generic message handling rules.
The "Content-Encoding" header field indicates what content-codings
have been applied to the representation beyond those inherent in the media
type, and thus what decoding mechanisms have to be applied in order to obtain
the media-type referenced by the Content-Type header field.
Content-Encoding is primarily used to allow a representation to be
compressed without losing the identity of its underlying media type.
Content-Encoding = 1#content-coding
Content codings are defined in . An example of its use is
Content-Encoding: gzip
The content-coding is a characteristic of the representation.
Typically, the representation body is stored with this
encoding and is only decoded before rendering or analogous usage.
However, a transforming proxy &MAY; modify the content-coding if the
new coding is known to be acceptable to the recipient, unless the
"no-transform" cache-control directive is present in the message.
If the media type includes an inherent encoding, such as a data format
that is always compressed, then that encoding would not be restated as
a Content-Encoding even if it happens to be the same algorithm as one
of the content-codings. Such a content-coding would only be listed if,
for some bizarre reason, it is applied a second time to form the
representation. Likewise, an origin server might choose to publish the
same payload data as multiple representations that differ only in whether
the coding is defined as part of Content-Type or
Content-Encoding, since some user agents will behave differently in their
handling of each response (e.g., open a "Save as ..." dialog instead of
automatic decompression and rendering of content).
A representation that has a content-coding applied to it &MUST; include
a Content-Encoding header field that lists the content-coding(s) applied.
If multiple encodings have been applied to a representation, the content
codings &MUST; be listed in the order in which they were applied.
Additional information about the encoding parameters &MAY; be provided
by other header fields not defined by this specification.
If the content-coding of a representation in a request message is not
acceptable to the origin server, the server &SHOULD; respond with a
status code of 415 (Unsupported Media Type).
The "Content-Language" header field describes the natural
language(s) of the intended audience for the representation. Note that this might
not be equivalent to all the languages used within the representation.
Content-Language = 1#language-tag
Language tags are defined in . The primary purpose of
Content-Language is to allow a user to identify and differentiate
representations according to the user's own preferred language. Thus, if the
body content is intended only for a Danish-literate audience, the
appropriate field is
Content-Language: da
If no Content-Language is specified, the default is that the content
is intended for all language audiences. This might mean that the
sender does not consider it to be specific to any natural language,
or that the sender does not know for which language it is intended.
Multiple languages &MAY; be listed for content that is intended for
multiple audiences. For example, a rendition of the "Treaty of
Waitangi", presented simultaneously in the original Maori and English
versions, would call for
Content-Language: mi, en
However, just because multiple languages are present within a representation
does not mean that it is intended for multiple linguistic audiences.
An example would be a beginner's language primer, such as "A First
Lesson in Latin", which is clearly intended to be used by an
English-literate audience. In this case, the Content-Language would
properly only include "en".
Content-Language &MAY; be applied to any media type — it is not
limited to textual documents.
The "Content-Location" header field supplies a URI that can be used
as a specific identifier for the representation in this message.
In other words, if one were to perform a GET on this URI at the time
of this message's generation, then a 200 (OK) response would
contain the same representation that is enclosed as payload in this message.
Content-Location = absolute-URI / partial-URI
The Content-Location value is not a replacement for the effective
Request URI (&effective-request-uri;). It is representation metadata.
It has the same syntax and semantics as the header field of the same name
defined for MIME body parts in .
However, its appearance in an HTTP message has some special implications
for HTTP recipients.
If Content-Location is included in a response message and its value
is the same as the effective request URI, then the response payload
&SHOULD; be considered a current representation of that resource.
For a GET or HEAD request, this is the same as the default semantics
when no Content-Location is provided by the server. For a state-changing
request like PUT or POST, it implies that the server's response contains
the new representation of that resource, thereby distinguishing it from
representations that might only report about the action (e.g., "It worked!").
This allows authoring applications to update their local copies without
the need for a subsequent GET request.
If Content-Location is included in a response message and its value
differs from the effective request URI, then the origin server is
informing recipients that this representation has its own, presumably
more specific, identifier. For a GET or HEAD request, this is an
indication that the effective request URI identifies a resource that
is subject to content negotiation and the selected representation for
this response can also be found at the identified URI. For other
methods, such a Content-Location indicates that this representation
contains a report on the action's status and the same report is
available (for future access with GET) at the given URI. For
example, a purchase transaction made via a POST request might
include a receipt document as the payload of the 200 (OK)
response; the Content-Location value provides an identifier for retrieving
a copy of that same receipt in the future.
If Content-Location is included in a request message, then it &MAY;
be interpreted by the origin server as an indication of where the
user agent originally obtained the content of the enclosed
representation (prior to any subsequent modification of the content
by that user agent). In other words, the user agent is providing
the same representation metadata that it received with the original
representation. However, such interpretation &MUST-NOT; be used to
alter the semantics of the method requested by the client. For
example, if a client makes a PUT request on a negotiated resource
and the origin server accepts that PUT (without redirection), then the
new set of values for that resource is expected to be consistent with
the one representation supplied in that PUT; the Content-Location
cannot be used as a form of reverse content selection that
identifies only one of the negotiated representations to be updated.
If the user agent had wanted the latter semantics, it would have applied
the PUT directly to the Content-Location URI.
A Content-Location field received in a request message is transitory
information that &SHOULD-NOT; be saved with other representation
metadata for use in later responses. The Content-Location's value
might be saved for use in other contexts, such as within source links
or other metadata.
A cache cannot assume that a representation with a Content-Location
different from the URI used to retrieve it can be used to respond to
later requests on that Content-Location URI.
If the Content-Location value is a partial URI, the partial URI is
interpreted relative to the effective request URI.
The "Content-Type" header field indicates the media type of the
representation. In the case of responses to the HEAD method, the media type is
that which would have been sent had the request been a GET.
Content-Type = media-type
Media types are defined in . An example of the field is
Content-Type: text/html; charset=ISO-8859-4
Further discussion of Content-Type is provided in .
The "Date" header field represents the date and time at which
the message was originated, having the same semantics as the Origination
Date Field (orig-date) defined in .
The field value is an HTTP-date, as defined in ;
it &MUST; be sent in rfc1123-date format.
Date = HTTP-date
An example is
Date: Tue, 15 Nov 1994 08:12:31 GMT
Origin servers &MUST; include a Date header field in all responses,
except in these cases:
If the response status code is 100 (Continue) or
101 (Switching Protocols), the response &MAY; include a
Date header field, at the server's option.If the response status code conveys a server error, e.g., 500
(Internal Server Error) or 503 (Service Unavailable),
and it is inconvenient or impossible to generate a valid Date.If the server does not have a clock that can provide a
reasonable approximation of the current time, its responses
&MUST-NOT; include a Date header field.
A received message that does not have a Date header field &MUST; be
assigned one by the recipient if the message will be cached by that
recipient.
Clients can use the Date header field as well; in order to keep request
messages small, they are advised not to include it when it doesn't convey
any useful information (as is usually the case for requests that do not
contain a payload).
The HTTP-date sent in a Date header field &SHOULD-NOT; represent a date and
time subsequent to the generation of the message. It &SHOULD; represent
the best available approximation of the date and time of message
generation, unless the implementation has no means of generating a
reasonably accurate date and time. In theory, the date ought to
represent the moment just before the payload is generated. In
practice, the date can be generated at any time during the message
origination without affecting its semantic value.
The "Expect" header field is used to indicate that particular
server behaviors are required by the client.
Expect = 1#expectationexpectation = expect-name [ BWS "=" BWSexpect-value ]
*( OWS ";" [ OWSexpect-param ] )
expect-param = expect-name [ BWS "=" BWSexpect-value ]
expect-name = tokenexpect-value = token / quoted-string
If all received Expect header field(s) are syntactically valid but contain
an expectation that the recipient does not understand or cannot comply with,
the recipient &MUST; respond with a 417 (Expectation Failed) status code. A
recipient of a syntactically invalid Expectation header field &MUST; respond
with a 4xx status code other than 417.
The only expectation defined by this specification is:
100-continue
The "100-continue" expectation is defined &use100;. It does not support
any expect-params.
Comparison is case-insensitive for names (expect-name), and case-sensitive
for values (expect-value).
The Expect mechanism is hop-by-hop: the above requirements apply to any
server, including proxies. However, the Expect header field itself is
end-to-end; it &MUST; be forwarded if the request is forwarded.
Many older HTTP/1.0 and HTTP/1.1 applications do not understand the Expect
header field.
The "From" header field, if given, &SHOULD; contain an Internet
e-mail address for the human user who controls the requesting user
agent. The address &SHOULD; be machine-usable, as defined by "mailbox"
in :
From = mailboxmailbox = <mailbox, defined in >
An example is:
From: webmaster@example.org
This header field &MAY; be used for logging purposes and as a means for
identifying the source of invalid or unwanted requests. It &SHOULD-NOT;
be used as an insecure form of access protection. The interpretation
of this field is that the request is being performed on behalf of the
person given, who accepts responsibility for the method performed. In
particular, robot agents &SHOULD; include this header field so that the
person responsible for running the robot can be contacted if problems
occur on the receiving end.
The Internet e-mail address in this field &MAY; be separate from the
Internet host which issued the request. For example, when a request
is passed through a proxy the original issuer's address &SHOULD; be
used.
The client &SHOULD-NOT; send the From header field without the user's
approval, as it might conflict with the user's privacy interests or
their site's security policy. It is strongly recommended that the
user be able to disable, enable, and modify the value of this field
at any time prior to a request.
The "Location" header field &MAY; be sent in responses to refer to
a specific resource in accordance with the semantics of the status
code.
Location = URI-reference
For 201 (Created) responses, the Location is the URI of the
new resource which was created by the request. For 3xx (Redirection)
responses, the location &SHOULD; indicate the server's preferred URI for
automatic redirection to the resource.
The field value consists of a single URI-reference. When it has the form
of a relative reference (),
the final value is computed by resolving it against the effective request
URI (). If the original URI, as
navigated to by the user agent, did contain a fragment identifier, and the
final value does not, then the original URI's fragment identifier is added
to the final value.
For example, the original URI "http://www.example.org/~tim", combined with a field value given as:
Location: /pub/WWW/People.html#tim
would result in a final value of "http://www.example.org/pub/WWW/People.html#tim"An original URI "http://www.example.org/index.html#larry", combined with a field value given as:
Location: http://www.example.net/index.html
would result in a final value of "http://www.example.net/index.html#larry", preserving the original fragment identifier.
&Note; Some recipients attempt to recover from Location fields
that are not valid URI references. This specification does not mandate or
define such processing, but does allow it.
There are circumstances in which a fragment identifier in a Location URI
would not be appropriate. For instance, when it appears in a 201
(Created) response, where the Location header field specifies the
URI for the entire created resource.
&Note; The Content-Location header field
(&header-content-location;) differs from Location in that the
Content-Location identifies the most specific resource corresponding to the
enclosed representation. It is therefore possible for a response to contain
header fields for both Location and Content-Location.
The "Max-Forwards" header field provides a mechanism with the
TRACE () and OPTIONS ()
methods to limit the number of times that the request is forwarded by
proxies. This can be useful when the client is attempting to
trace a request which appears to be failing or looping mid-chain.
Max-Forwards = 1*DIGIT
The Max-Forwards value is a decimal integer indicating the remaining
number of times this request message can be forwarded.
Each recipient of a TRACE or OPTIONS request
containing a Max-Forwards header field &MUST; check and update its
value prior to forwarding the request. If the received value is zero
(0), the recipient &MUST-NOT; forward the request; instead, it &MUST;
respond as the final recipient. If the received Max-Forwards value is
greater than zero, then the forwarded message &MUST; contain an updated
Max-Forwards field with a value decremented by one (1).
The Max-Forwards header field &MAY; be ignored for all other request
methods.
The "Referer" [sic] header field allows the client to specify the
URI of the resource from which the target URI was obtained (the
"referrer", although the header field is misspelled.).
The Referer header field allows servers to generate lists of back-links to
resources for interest, logging, optimized caching, etc. It also allows
obsolete or mistyped links to be traced for maintenance. Some servers use
Referer as a means of controlling where they allow links from (so-called
"deep linking"), but legitimate requests do not always
contain a Referer header field.
If the target URI was obtained from a source that does not have its own
URI (e.g., input from the user keyboard), the Referer field &MUST; either be
sent with the value "about:blank", or not be sent at all. Note that this
requirement does not apply to sources with non-HTTP URIs (e.g., FTP).
Referer = absolute-URI / partial-URI
Example:
Referer: http://www.example.org/hypertext/Overview.html
If the field value is a relative URI, it &SHOULD; be interpreted
relative to the effective request URI. The URI &MUST-NOT; include a fragment. See
for security considerations.
The header "Retry-After" field can be used with a 503 (Service
Unavailable) response to indicate how long the service is expected to
be unavailable to the requesting client. This field &MAY; also be used
with any 3xx (Redirection) response to indicate the minimum time the
user-agent is asked to wait before issuing the redirected request.
The value of this field can be either an HTTP-date or an integer number
of seconds (in decimal) after the time of the response.
Retry-After = HTTP-date / delta-seconds
Time spans are non-negative decimal integers, representing time in
seconds.
delta-seconds = 1*DIGIT
Two examples of its use are
Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
Retry-After: 120
In the latter example, the delay is 2 minutes.
The "Server" header field contains information about the
software used by the origin server to handle the request.
The field can contain multiple
product tokens () and
comments (&header-fields;) identifying the server and any significant
subproducts. The product tokens are listed in order of their significance
for identifying the application.
Server = product *( RWS ( product / comment ) )
Example:
Server: CERN/3.0 libwww/2.17
If the response is being forwarded through a proxy, the proxy application
&MUST-NOT; modify the Server header field. Instead, it
&MUST; include a Via field (as described in &header-via;).
&Note; Revealing the specific software version of the server might
allow the server machine to become more vulnerable to attacks
against software that is known to contain security holes. Server
implementers are encouraged to make this field a configurable
option.
The "User-Agent" header field contains information about the user
agent originating the request. User agents &SHOULD; include this field with
requests.
Typically, it is used for statistical purposes, the tracing of protocol
violations, and tailoring responses to avoid particular user agent
limitations.
The field can contain multiple
product tokens ()
and comments (&header-fields;) identifying the agent and its
significant subproducts. By convention, the product tokens are listed in
order of their significance for identifying the application.
Because this field is usually sent on every request a user agent makes,
implementations are encouraged not to include needlessly fine-grained
detail, and to limit (or even prohibit) the addition of subproducts by third
parties. Overly long and detailed User-Agent field values make requests
larger and can also be used to identify ("fingerprint") the user against
their wishes.
Likewise, implementations are encouraged not to use the product tokens of
other implementations in order to declare compatibility with them, as this
circumvents the purpose of the field. Finally, they are encouraged not to
use comments to identify products; doing so makes the field value more
difficult to parse.
User-Agent = product *( RWS ( product / comment ) )
Example:
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
The registration procedure for HTTP request methods is defined by
of this document.
The HTTP Method Registry shall be created at
and be populated with the registrations below:
MethodSafeIdempotentReferenceCONNECTnonoDELETEnoyesGETyesyesHEADyesyesOPTIONSyesyesPOSTnonoPUTnoyesTRACEyesyes
The registration procedure for HTTP Status Codes — previously defined
in — is now defined
by of this document.
The HTTP Status Code Registry located at
shall be updated with the registrations below:
ValueDescriptionReference100Continue101Switching Protocols200OK201Created202Accepted203Non-Authoritative Information204No Content205Reset Content300Multiple Choices301Moved Permanently302Found303See Other305Use Proxy306(Unused)307Temporary Redirect400Bad Request402Payment Required403Forbidden404Not Found405Method Not Allowed406Not Acceptable408Request Timeout409Conflict410Gone411Length Required413Request Representation Too Large414URI Too Long415Unsupported Media Type417Expectation Failed426Upgrade Required500Internal Server Error501Not Implemented502Bad Gateway503Service Unavailable504Gateway Timeout505HTTP Version Not Supported
The Message Header Field Registry located at shall be updated
with the permanent registrations below (see ):
Header Field NameProtocolStatusReferenceAccepthttpstandardAccept-CharsethttpstandardAccept-EncodinghttpstandardAccept-LanguagehttpstandardAllowhttpstandardContent-EncodinghttpstandardContent-LanguagehttpstandardContent-LocationhttpstandardContent-TypehttpstandardDatehttpstandardExpecthttpstandardFromhttpstandardLocationhttpstandardMIME-VersionhttpstandardMax-ForwardshttpstandardRefererhttpstandardRetry-AfterhttpstandardServerhttpstandardUser-Agenthttpstandard
The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
The registration procedure for HTTP Content Codings is now defined
by of this document.
The HTTP Content Codings Registry located at
shall be updated with the registration below:
NameDescriptionReferencecompressUNIX "compress" program method
&compress-coding;
deflate"deflate" compression mechanism () used inside
the "zlib" data format ()
&deflate-coding;
gzipSame as GNU zip
&gzip-coding;
identityreserved (synonym for "no encoding" in Accept-Encoding
header field)
This section is meant to inform application developers, information
providers, and users of the security limitations in HTTP/1.1 as
described by this document. The discussion does not include
definitive solutions to the problems revealed, though it does make
some suggestions for reducing security risks.
Like any generic data transfer protocol, HTTP cannot regulate the
content of the data that is transferred, nor is there any a priori
method of determining the sensitivity of any particular piece of
information within the context of any given request. Therefore,
applications &SHOULD; supply as much control over this information as
possible to the provider of that information. Four header fields are
worth special mention in this context: Server,
Via, Referer and From.
Revealing the specific software version of the server might allow the
server machine to become more vulnerable to attacks against software
that is known to contain security holes. Implementers &SHOULD; make the
Server header field a configurable option.
Proxies which serve as a portal through a network firewall &SHOULD;
take special precautions regarding the transfer of header information
that identifies the hosts behind the firewall. In particular, they
&SHOULD; remove, or replace with sanitized versions, any Via
fields generated behind the firewall.
The Referer header field allows reading patterns to be
studied and reverse links drawn. Although it can be very useful, its power
can be abused if user details are not separated from the information
contained in the Referer. Even when the personal information has been
removed, the Referer header field might indicate a private document's URI
whose publication would be inappropriate.
The information sent in the From field might conflict with
the user's privacy interests or their site's security policy, and hence it
&SHOULD-NOT; be transmitted without the user being able to disable,
enable, and modify the contents of the field. The user &MUST; be able
to set the contents of this field within a user preference or
application defaults configuration.
We suggest, though do not require, that a convenient toggle interface
be provided for the user to enable or disable the sending of
From and Referer information.
The User-Agent () or
Server () header fields can
sometimes be used to determine that a specific client or server has a
particular security hole which might be exploited. Unfortunately, this same
information is often used for other valuable purposes for which HTTP
currently has no better mechanism.
Furthermore, the User-Agent header field might contain enough
entropy to be used, possibly in conjunction with other material, to uniquely
identify the user.
Some request methods, like TRACE (), expose information
that was sent in request header fields within the body of their response.
Clients &SHOULD; be careful with sensitive information, like Cookies,
Authorization credentials, and other header fields that might be used to
collect data from the client.
Because the source of a link might be private information or might
reveal an otherwise private information source, it is strongly
recommended that the user be able to select whether or not the
Referer field is sent. For example, a browser client could
have a toggle switch for browsing openly/anonymously, which would
respectively enable/disable the sending of Referer and From
information.
Clients &SHOULD-NOT; include a Referer header field in a
(non-secure) HTTP request if the referring page was transferred with a secure
protocol.
Authors of services &SHOULD-NOT; use GET-based forms for the submission of
sensitive data because that data will be placed in the request-target. Many
existing servers, proxies, and user agents log or display the request-target
in places where it might be visible to third parties. Such services can
use POST-based form submission instead.
If a single server supports multiple organizations that do not trust
one another, then it &MUST; check the values of Location and
Content-Location header fields in responses that are
generated under control of said organizations to make sure that they do not
attempt to invalidate resources over which they have no authority.
Furthermore, appending the fragment identifier from one URI to another
one obtained from a Location header field might leak
confidential information to the target server — although the fragment
identifier is not transmitted in the final request, it might be visible to
the user agent through other means, such as scripting.
Since tunneled data is opaque to the proxy, there are additional
risks to tunneling to other well-known or reserved ports.
A HTTP client CONNECTing to port 25 could relay spam
via SMTP, for example. As such, proxies &SHOULD; restrict CONNECT
access to a small number of known ports.
Accept header fields can reveal information about the user to all
servers which are accessed. The Accept-Language header field
in particular can reveal information the user would consider to be of a
private nature, because the understanding of particular languages is often
strongly correlated to the membership of a particular ethnic group.
User agents which offer the option to configure the contents of an
Accept-Language header field to be sent in every request are strongly
encouraged to let the configuration process include a message which
makes the user aware of the loss of privacy involved.
An approach that limits the loss of privacy would be for a user agent
to omit the sending of Accept-Language header fields by default, and to ask
the user whether or not to start sending Accept-Language header fields to a
server if it detects, by looking for any Vary header fields
generated by the server, that such sending could improve the quality
of service.
Elaborate user-customized accept header fields sent in every request,
in particular if these include quality values, can be used by servers
as relatively reliable and long-lived user identifiers. Such user
identifiers would allow content providers to do click-trail tracking,
and would allow collaborating content providers to match cross-server
click-trails or form submissions of individual users. Note that for
many users not behind a proxy, the network address of the host
running the user agent will also serve as a long-lived user
identifier. In environments where proxies are used to enhance
privacy, user agents ought to be conservative in offering accept
header field configuration options to end users. As an extreme privacy
measure, proxies could filter the accept header fields in relayed requests.
General purpose user agents which provide a high degree of header field
configurability &SHOULD; warn users about the loss of privacy which can
be involved.
See &acks;.
HTTP/1.1, part 1: Message Routing and Syntax"Adobe Systems Incorporatedfielding@gbiv.comWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.deConnectionContent-LengthTransfer-EncodingUpgradeViaHTTP/1.1, part 4: Conditional RequestsAdobe Systems Incorporatedfielding@gbiv.comWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.de304 (Not Modified)ETagIf-MatchIf-Modified-SinceIf-None-MatchIf-Unmodified-SinceHTTP/1.1, part 5: Range RequestsAdobe Systems Incorporatedfielding@gbiv.comWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.de206206 (Partial Content)Content-RangeIf-RangeRangeHTTP/1.1, part 6: CachingAdobe Systems Incorporatedfielding@gbiv.comWorld Wide Web Consortiumylafon@w3.orgRackspacemnot@mnot.netgreenbytes GmbHjulian.reschke@greenbytes.deVaryHTTP/1.1, part 7: AuthenticationAdobe Systems Incorporatedfielding@gbiv.comWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.deZLIB Compressed Data Format Specification version 3.3Aladdin Enterprisesghost@aladdin.comDEFLATE Compressed Data Format Specification version 1.3Aladdin Enterprisesghost@aladdin.comGZIP file format specification version 4.3Aladdin Enterprisesghost@aladdin.comgzip@prep.ai.mit.edumadler@alumni.caltech.edughost@aladdin.comrandeg@alumni.rpi.eduMultipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message BodiesInnosoft International, Inc.ned@innosoft.comFirst Virtual Holdingsnsb@nsb.fv.comMultipurpose Internet Mail Extensions (MIME) Part Two: Media TypesInnosoft International, Inc.ned@innosoft.comFirst Virtual Holdingsnsb@nsb.fv.comKey words for use in RFCs to Indicate Requirement LevelsHarvard Universitysob@harvard.eduUniform Resource Identifier (URI): Generic SyntaxWorld Wide Web Consortiumtimbl@w3.orghttp://www.w3.org/People/Berners-Lee/Day Softwarefielding@gbiv.comhttp://roy.gbiv.com/Adobe Systems IncorporatedLMM@acm.orghttp://larry.masinter.net/Matching of Language TagsYahoo! Inc.addison@inter-locale.comGooglemark.davis@macchiato.comAugmented BNF for Syntax Specifications: ABNFBrandenburg InternetWorkingdcrocker@bbiw.netTHUS plc.paul.overell@thus.netTags for Identifying LanguagesLab126addison@inter-locale.comGooglemark.davis@google.comRequirements for Internet Hosts - Application and SupportUniversity of Southern California (USC), Information Sciences InstituteBraden@ISI.EDUHypertext Transfer Protocol -- HTTP/1.0MIT, Laboratory for Computer Sciencetimbl@w3.orgUniversity of California, Irvine, Department of Information and Computer Sciencefielding@ics.uci.eduW3 Consortium, MIT Laboratory for Computer Sciencefrystyk@w3.orgMultipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and ExamplesInnosoft International, Inc.ned@innosoft.comFirst Virtual Holdingsnsb@nsb.fv.comHypertext Transfer Protocol -- HTTP/1.1University of California, Irvine, Department of Information and Computer Sciencefielding@ics.uci.eduMIT Laboratory for Computer Sciencejg@w3.orgDigital Equipment Corporation, Western Research Laboratorymogul@wrl.dec.comMIT Laboratory for Computer Sciencefrystyk@w3.orgMIT Laboratory for Computer Sciencetimbl@w3.orgCommon Internet Message HeadersStockholm University/KTHjpalme@dsv.su.seIETF Policy on Character Sets and LanguagesUNINETTHarald.T.Alvestrand@uninett.noTransparent Content Negotiation in HTTPTechnische Universiteit Eindhovenkoen@win.tue.nlHewlett-Packard Companymutz@hpl.hp.comReturning Values from Forms: multipart/form-dataXerox Palo Alto Research Centermasinter@parc.xerox.comMIME Encapsulation of Aggregate Documents, such as HTML (MHTML)Stockholm University and KTHjpalme@dsv.su.seMicrosoft Corporationalexhop@microsoft.comLotus Development CorporationShelness@lotus.comstef@nma.comHypertext Transfer Protocol -- HTTP/1.1University of California, Irvinefielding@ics.uci.eduW3Cjg@w3.orgCompaq Computer Corporationmogul@wrl.dec.comMIT Laboratory for Computer Sciencefrystyk@w3.orgXerox Corporationmasinter@parc.xerox.comMicrosoft Corporationpaulle@microsoft.comW3Ctimbl@w3.orgUpgrading to TLS Within HTTP/1.14K Associates / UC Irvinerohit@4K-associates.comAgranat Systems, Inc.lawrence@agranat.comUTF-8, a transformation format of ISO 10646Alis Technologiesfyergeau@alis.comRegistration Procedures for Message Header FieldsNine by NineGK-IETF@ninebynine.orgBEA Systemsmnot@pobox.comHP LabsJeffMogul@acm.orgMedia Type Specifications and Registration ProceduresSun Microsystemsned.freed@mrochek.comklensin+ietf@jck.comGuidelines for Writing an IANA Considerations Section in RFCsIBMnarten@us.ibm.comGoogleHarald@Alvestrand.noInternet Message FormatQualcomm IncorporatedPATCH Method for HTTPLinden LabCharacter Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parametersgreenbytes GmbHHafenweg 16MuensterNW48155Germanyjulian.reschke@greenbytes.dehttp://greenbytes.de/tech/webdav/Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 AlgorithmsUse of the Content-Disposition Header Field
in the Hypertext Transfer Protocol (HTTP)greenbytes GmbHjulian.reschke@greenbytes.deThe Hypertext Transfer Protocol (HTTP) Status Code 308 (Permanent Redirect)greenbytes GmbHjulian.reschke@greenbytes.de
HTTP/1.1 uses many of the constructs defined for Internet Mail () and the Multipurpose Internet Mail Extensions (MIME ) to
allow a message body to be transmitted in an open variety of
representations and with extensible mechanisms. However, RFC 2045
discusses mail, and HTTP has a few features that are different from
those described in MIME. These differences were carefully chosen
to optimize performance over binary connections, to allow greater
freedom in the use of new media types, to make date comparisons
easier, and to acknowledge the practice of some early HTTP servers
and clients.
This appendix describes specific areas where HTTP differs from MIME.
Proxies and gateways to strict MIME environments &SHOULD; be
aware of these differences and provide the appropriate conversions
where necessary. Proxies and gateways from MIME environments to HTTP
also need to be aware of the differences because some conversions
might be required.
HTTP is not a MIME-compliant protocol. However, HTTP/1.1 messages &MAY;
include a single MIME-Version header field to indicate what
version of the MIME protocol was used to construct the message. Use
of the MIME-Version header field indicates that the message is in
full conformance with the MIME protocol (as defined in ).
Proxies/gateways are responsible for ensuring full conformance (where
possible) when exporting HTTP messages to strict MIME environments.
MIME-Version = 1*DIGIT "." 1*DIGIT
MIME version "1.0" is the default for use in HTTP/1.1. However,
HTTP/1.1 message parsing and semantics are defined by this document
and not the MIME specification.
MIME requires that an Internet mail body-part be converted to
canonical form prior to being transferred, as described in .
of this document describes the forms
allowed for subtypes of the "text" media type when transmitted over
HTTP. requires that content with a type of "text" represent
line breaks as CRLF and forbids the use of CR or LF outside of line
break sequences. HTTP allows CRLF, bare CR, and bare LF to indicate a
line break within text content when a message is transmitted over
HTTP.
Where it is possible, a proxy or gateway from HTTP to a strict MIME
environment &SHOULD; translate all line breaks within the text media
types described in
of this document to the RFC 2049 canonical form of CRLF. Note, however, that
this might be complicated by the presence of a Content-Encoding
and by the fact that HTTP allows the use of some character encodings which do
not use octets 13 and 10 to represent CR and LF, respectively, as is the case
for some multi-byte character encodings.
Conversion will break any cryptographic
checksums applied to the original content unless the original content
is already in canonical form. Therefore, the canonical form is
recommended for any content that uses such checksums in HTTP.
HTTP/1.1 uses a restricted set of date formats (&http-date;) to
simplify the process of date comparison. Proxies and gateways from
other protocols &SHOULD; ensure that any Date header field
present in a message conforms to one of the HTTP/1.1 formats and rewrite
the date if necessary.
MIME does not include any concept equivalent to HTTP/1.1's
Content-Encoding header field. Since this acts as a modifier
on the media type, proxies and gateways from HTTP to MIME-compliant
protocols &MUST; either change the value of the Content-Type
header field or decode the representation before forwarding the message.
(Some experimental applications of Content-Type for Internet mail have used
a media-type parameter of ";conversions=<content-coding>" to perform
a function equivalent to Content-Encoding. However, this parameter is
not part of the MIME standards).
HTTP does not use the Content-Transfer-Encoding field of MIME.
Proxies and gateways from MIME-compliant protocols to HTTP &MUST;
remove any Content-Transfer-Encoding
prior to delivering the response message to an HTTP client.
Proxies and gateways from HTTP to MIME-compliant protocols are
responsible for ensuring that the message is in the correct format
and encoding for safe transport on that protocol, where "safe
transport" is defined by the limitations of the protocol being used.
Such a proxy or gateway &SHOULD; label the data with an appropriate
Content-Transfer-Encoding if doing so will improve the likelihood of
safe transport over the destination protocol.
HTTP implementations which share code with MHTML implementations
need to be aware of MIME line length limitations. Since HTTP does not
have this limitation, HTTP does not fold long lines. MHTML messages
being transported by HTTP follow all conventions of MHTML, including
line length limitations and folding, canonicalization, etc., since
HTTP transports all message-bodies as payload (see ) and
does not interpret the content or any MIME header lines that might be
contained therein.
and document protocol elements used by some
existing HTTP implementations, but not consistently and correctly
across most HTTP/1.1 applications. Implementers are advised to be
aware of these features, but cannot rely upon their presence in, or
interoperability with, other HTTP/1.1 applications. Some of these
describe proposed experimental features, and some describe features
that experimental deployment found lacking that are now addressed in
the base HTTP/1.1 specification.
A number of other header fields, such as Content-Disposition and Title,
from SMTP and MIME are also often implemented (see
and ).
Introduce Method Registry.
()
Clarify definition of POST.
()
Remove requirement to handle all Content-* header fields; ban use of
Content-Range with PUT.
()
Take over definition of CONNECT method from .
()
Take over the Status Code Registry, previously defined in
.
()
Broadened the definition of 203 (Non-Authoritative Information)
to include cases of payload transformations as well.
()
Status codes 301, 302, and 307:
removed the normative requirements on both response payloads and user interaction.
()
Failed to consider that there are many other request methods that are safe
to automatically redirect, and further that the user agent is able to make
that determination based on the request method semantics.
Furthermore, allow user agents to rewrite the method from POST to GET
for status codes 301 and 302.
(Sections ,
and
)
Deprecate 305 (Use Proxy) status code, because user agents did
not implement it. It used to indicate that the target resource needs to be
accessed through the proxy given by the Location field. The
Location field gave the URI of the proxy. The recipient was expected to
repeat this single request via the proxy.
()
Define status 426 (Upgrade Required) (this was incorporated
from ).
()
Change ABNF productions for header fields to only define the field value.
()
Reclassify "Allow" as response header field, removing the
option to specify it in a PUT request.
Relax the server requirement on the contents of the Allow header field and
remove requirement on clients to always trust the header field value.
()
The ABNF for the Expect header field has been both fixed
(allowing parameters for value-less expectations as well) and simplified
(allowing trailing semicolons after "100-continue" when they were invalid
before).
()
Correct syntax of Location header field to allow URI
references (including relative references and fragments), as referred symbol
"absoluteURI" wasn't what was expected, and add some clarifications as to
when use of fragments would not be appropriate.
()
Restrict Max-Forwards header field to OPTIONS and TRACE
(previously, extension methods could have used it as well).
()
Allow Referer field value of "about:blank" as alternative to
not specifying it.
()
In the description of the Server header field, the
Via field was described as a SHOULD. The requirement was and
is stated correctly in the description of the Via header field in
&header-via;.
()
Clarify contexts that charset is used in.
()
Registration of Content Codings now requires IETF Review
()
Remove the default character encoding of "ISO-8859-1" for text media types; the
default now is whatever the media type definition says.
()
Change ABNF productions for header fields to only define the field value.
()
Remove definition of Content-MD5 header field because it was inconsistently
implemented with respect to partial responses, and also because of known
deficiencies in the hash algorithm itself (see for details).
()
Remove ISO-8859-1 special-casing in Accept-Charset.
()
Remove base URI setting semantics for Content-Location due to
poor implementation support, which was caused by too many broken servers emitting
bogus Content-Location header fields, and also the potentially undesirable effect
of potentially breaking relative links in content-negotiated resources.
()
Remove reference to non-existant identity transfer-coding value tokens.
()
Remove discussion of Content-Disposition header field, it is now defined
by .
()
The following core rules are included by
reference, as defined in :
ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
DIGIT (decimal 0-9), DQUOTE (double quote),
HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
OCTET (any 8-bit sequence of data), SP (space), and
VCHAR (any visible US-ASCII character).
The rules below are defined in :
BWS = <BWS, defined in &whitespace;>
OWS = <OWS, defined in &whitespace;>
RWS = <RWS, defined in &whitespace;>
quoted-string = <quoted-string, defined in &field-components;>
token = <token, defined in &field-components;>
word = <word, defined in &field-components;>
absolute-URI = <absolute-URI, defined in &uri;>
comment = <comment, defined in &field-components;>
partial-URI = <partial-URI, defined in &uri;>
qvalue = <qvalue, defined in &qvalue;>
URI-reference = <URI-reference, defined in &uri;>
Accept = [ ( "," / ( media-range [ accept-params ] ) ) *( OWS "," [
OWS ( media-range [ accept-params ] ) ] ) ]
Accept-Charset = *( "," OWS ) ( ( charset / "*" ) [ OWS ";" OWS "q="
qvalue ] ) *( OWS "," [ OWS ( ( charset / "*" ) [ OWS ";" OWS "q="
qvalue ] ) ] )
Accept-Encoding = [ ( "," / ( codings [ OWS ";" OWS "q=" qvalue ] ) )
*( OWS "," [ OWS ( codings [ OWS ";" OWS "q=" qvalue ] ) ] ) ]
Accept-Language = *( "," OWS ) ( language-range [ OWS ";" OWS "q="
qvalue ] ) *( OWS "," [ OWS ( language-range [ OWS ";" OWS "q="
qvalue ] ) ] )
Allow = [ ( "," / method ) *( OWS "," [ OWS method ] ) ]
BWS = <BWS, defined in [Part1], Section 3.2.1>
Content-Encoding = *( "," OWS ) content-coding *( OWS "," [ OWS
content-coding ] )
Content-Language = *( "," OWS ) language-tag *( OWS "," [ OWS
language-tag ] )
Content-Location = absolute-URI / partial-URI
Content-Type = media-type
Date = HTTP-date
Expect = *( "," OWS ) expectation *( OWS "," [ OWS expectation ] )
From = mailbox
GMT = %x47.4D.54 ; GMT
HTTP-date = rfc1123-date / obs-date
Location = URI-reference
MIME-Version = 1*DIGIT "." 1*DIGIT
Max-Forwards = 1*DIGIT
OWS = <OWS, defined in [Part1], Section 3.2.1>
RWS = <RWS, defined in [Part1], Section 3.2.1>
Referer = absolute-URI / partial-URI
Retry-After = HTTP-date / delta-seconds
Server = product *( RWS ( product / comment ) )
URI-reference = <URI-reference, defined in [Part1], Section 2.8>
User-Agent = product *( RWS ( product / comment ) )
absolute-URI = <absolute-URI, defined in [Part1], Section 2.8>
accept-ext = OWS ";" OWS token [ "=" word ]
accept-params = OWS ";" OWS "q=" qvalue *accept-ext
asctime-date = day-name SP date3 SP time-of-day SP year
attribute = token
charset = token
codings = content-coding / "identity" / "*"
comment = <comment, defined in [Part1], Section 3.2.4>
content-coding = token
date1 = day SP month SP year
date2 = day "-" month "-" 2DIGIT
date3 = month SP ( 2DIGIT / ( SP DIGIT ) )
day = 2DIGIT
day-name = %x4D.6F.6E ; Mon
/ %x54.75.65 ; Tue
/ %x57.65.64 ; Wed
/ %x54.68.75 ; Thu
/ %x46.72.69 ; Fri
/ %x53.61.74 ; Sat
/ %x53.75.6E ; Sun
day-name-l = %x4D.6F.6E.64.61.79 ; Monday
/ %x54.75.65.73.64.61.79 ; Tuesday
/ %x57.65.64.6E.65.73.64.61.79 ; Wednesday
/ %x54.68.75.72.73.64.61.79 ; Thursday
/ %x46.72.69.64.61.79 ; Friday
/ %x53.61.74.75.72.64.61.79 ; Saturday
/ %x53.75.6E.64.61.79 ; Sunday
delta-seconds = 1*DIGIT
expect-name = token
expect-param = expect-name [ BWS "=" BWS expect-value ]
expect-value = token / quoted-string
expectation = expect-name [ BWS "=" BWS expect-value ] *( OWS ";" [
OWS expect-param ] )
hour = 2DIGIT
language-range = <language-range, defined in [RFC4647], Section 2.1>
language-tag = <Language-Tag, defined in [RFC5646], Section 2.1>
mailbox = <mailbox, defined in [RFC5322], Section 3.4>
media-range = ( "*/*" / ( type "/*" ) / ( type "/" subtype ) ) *( OWS
";" OWS parameter )
media-type = type "/" subtype *( OWS ";" OWS parameter )
method = token
minute = 2DIGIT
month = %x4A.61.6E ; Jan
/ %x46.65.62 ; Feb
/ %x4D.61.72 ; Mar
/ %x41.70.72 ; Apr
/ %x4D.61.79 ; May
/ %x4A.75.6E ; Jun
/ %x4A.75.6C ; Jul
/ %x41.75.67 ; Aug
/ %x53.65.70 ; Sep
/ %x4F.63.74 ; Oct
/ %x4E.6F.76 ; Nov
/ %x44.65.63 ; Dec
obs-date = rfc850-date / asctime-date
parameter = attribute "=" value
partial-URI = <partial-URI, defined in [Part1], Section 2.8>
product = token [ "/" product-version ]
product-version = token
quoted-string = <quoted-string, defined in [Part1], Section 3.2.4>
qvalue = <qvalue, defined in [Part1], Section 4.3.1>
rfc1123-date = day-name "," SP date1 SP time-of-day SP GMT
rfc850-date = day-name-l "," SP date2 SP time-of-day SP GMT
second = 2DIGIT
subtype = token
time-of-day = hour ":" minute ":" second
token = <token, defined in [Part1], Section 3.2.4>
type = token
value = word
word = <word, defined in [Part1], Section 3.2.4>
year = 4DIGIT
Extracted relevant partitions from .
Closed issues:
:
"Via is a MUST"
()
:
"Fragments allowed in Location"
()
:
"Safe Methods vs Redirection"
()
:
"Revise description of the POST method"
()
:
"Normative and Informative references"
:
"RFC2606 Compliance"
:
"Informative references"
:
"Redundant cross-references"
Other changes:
Move definitions of 304 and 412 condition codes to
Closed issues:
:
"Media Type Registrations"
()
:
"Clarification regarding quoting of charset values"
()
:
"Remove 'identity' token references"
()
:
"Accept-Encoding BNF"
:
"Normative and Informative references"
:
"RFC1700 references"
:
"Updating to RFC4288"
:
"Informative references"
:
"ISO-8859-1 Reference"
:
"Encoding References Normative"
:
"Normative up-to-date references"
Closed issues:
:
"PUT side effects"
:
"Duplicate Host header requirements"
Ongoing work on ABNF conversion ():
Move "Product Tokens" section (back) into Part 1, as "token" is used
in the definition of the Upgrade header field.
Add explicit references to BNF syntax and rules imported from other parts of the specification.
Copy definition of delta-seconds from Part6 instead of referencing it.
Ongoing work on ABNF conversion ():
Add explicit references to BNF syntax and rules imported from other parts of the specification.
Closed issues:
:
"Requiring Allow in 405 responses"
:
"Status Code Registry"
:
"Redirection vs. Location"
:
"Cacheability of 303 response"
:
"305 Use Proxy"
:
"Classification for Allow header field"
:
"PUT - 'store under' vs 'store at'"
Ongoing work on IANA Message Header Field Registration ():
Reference RFC 3984, and update header field registrations for header
fields defined in this document.
Ongoing work on ABNF conversion ():
Replace string literals when the string really is case-sensitive (method).
Closed issues:
:
"Quoting Charsets"
:
"Classification for Allow header field"
:
"missing default for qvalue in description of Accept-Encoding"
Ongoing work on IANA Message Header Field Registration ():
Reference RFC 3984, and update header field registrations for header
fields defined in this document.
Closed issues:
:
"OPTIONS request bodies"
:
"Description of CONNECT should refer to RFC2817"
:
"Location Content-Location reference request/response mixup"
Ongoing work on Method Registry ():
Added initial proposal for registration process, plus initial
content (non-HTTP/1.1 methods to be added by a separate specification).
Closed issues:
:
"Quoting Charsets"
:
"language tag matching (Accept-Language) vs RFC4647"
:
"RFC 1806 has been replaced by RFC2183"
Other changes:
:
"Encoding References Normative" — rephrase the annotation and reference
BCP97.
Closed issues:
:
"Content-*"
:
"RFC 2822 is updated by RFC 5322"
Ongoing work on ABNF conversion ():
Use "/" instead of "|" for alternatives.
Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
whitespace ("OWS") and required whitespace ("RWS").
Rewrite ABNFs to spell out whitespace rules, factor out
header field value format definitions.
Closed issues:
:
"RFC 2822 is updated by RFC 5322"
Ongoing work on ABNF conversion ():
Use "/" instead of "|" for alternatives.
Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
whitespace ("OWS") and required whitespace ("RWS").
Rewrite ABNFs to spell out whitespace rules, factor out
header field value format definitions.
Closed issues:
:
"reason-phrase BNF"
Final work on ABNF conversion ():
Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
Closed issues:
:
"Join "Differences Between HTTP Entities and RFC 2045 Entities"?"
Final work on ABNF conversion ():
Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
Other changes:
Move definition of quality values into Part 1.
Closed issues:
:
"Clarify when Referer is sent"
:
"status codes vs methods"
:
"Do not require "updates" relation for specs that register status codes or method names"
Closed issues:
:
"Content-Location isn't special"
:
"Content Sniffing"
Closed issues:
:
"Idempotency"
:
"TRACE security considerations"
:
"Clarify rules for determining what entities a response carries"
:
"update note citing RFC 1945 and 2068"
:
"update note about redirect limit"
:
"Location header field ABNF should use 'URI'"
:
"fragments in Location vs status 303"
:
"move IANA registrations for optional status codes"
Partly resolved issues:
:
"Are OPTIONS and TRACE safe?"
Closed issues:
:
"Updated reference for language tags"
:
"Clarify rules for determining what entities a response carries"
:
"Content-Location base-setting problems"
:
"Content Sniffing"
:
"pick IANA policy (RFC5226) for Transfer Coding / Content Coding"
:
"move definitions of gzip/deflate/compress to part 1"
Partly resolved issues:
:
"update IANA requirements wrt Transfer-Coding values" (add the
IANA Considerations subsection)
:
"update IANA requirements wrt Content-Coding values" (add the
IANA Considerations subsection)
Closed issues:
:
"Safe Methods vs Redirection" (we missed the introduction to the 3xx
status codes when fixing this previously)
Closed issues:
:
"Content Negotiation for media types"
:
"Accept-Language: which RFC4647 filtering?"
Closed issues:
:
"Fragment combination / precedence during redirects"
Partly resolved issues:
:
"Location header field payload handling"
:
"Term for the requested resource's URI"
Closed issues:
:
"MIME-Version not listed in P1, general header fields"
:
"IANA registry for content/transfer encodings"
:
"Content Sniffing"
:
"use of term "word" when talking about header field structure"
Partly resolved issues:
:
"Term for the requested resource's URI"
Closed issues:
:
"Clarify 'Requested Variant'"
:
"Clarify entity / representation / variant terminology"
:
"Methods and Caching"
:
"OPTIONS vs Max-Forwards"
:
"Status codes and caching"
:
"consider removing the 'changes from 2068' sections"
Closed issues:
:
"Clarify 'Requested Variant'"
:
"Content-Location isn't special"
:
"Delimiting messages with multipart/byteranges"
:
"Clarify entity / representation / variant terminology"
:
"confusing req. language for Content-Location"
:
"Content-Location on 304 responses"
:
"'requested resource' in content-encoding definition"
:
"consider removing the 'changes from 2068' sections"
Partly resolved issues:
:
"Content-MD5 and partial responses"
Closed issues:
:
"Considerations for new status codes"
:
"Considerations for new methods"
:
"User-Agent guidelines" (relating to the 'User-Agent' header field)
Closed issues:
:
"Factor out Content-Disposition"
Closed issues:
:
"Fragment combination / precedence during redirects" (added warning
about having a fragid on the redirect might cause inconvenience in
some cases)
:
"Content-* vs. PUT"
:
"205 Bodies"
:
"Understanding Content-* on non-PUT requests"
:
"Content-*"
:
"Header field type defaulting"
:
"PUT - 'store under' vs 'store at'"
:
"duplicate ABNF for reason-phrase"
:
"Note special status of Content-* prefix in header field registration procedures"
:
"Max-Forwards vs extension methods"
:
"What is the value space of HTTP status codes?" (actually fixed in
draft-ietf-httpbis-p2-semantics-11)
:
"Header Field Classification"
:
"PUT side effect: invalidation or just stale?"
:
"proxies not supporting certain methods"
:
"Migrate CONNECT from RFC2817 to p2"
:
"Migrate Upgrade details from RFC2817"
:
"clarify PUT semantics'"
:
"duplicate ABNF for 'Method'"
:
"untangle ABNFs for header fields"
Closed issues:
:
"Header Field Classification"
:
"untangle ABNFs for header fields"
:
"potentially misleading MAY in media-type def"
Closed issues:
:
"untangle ABNFs for header fields"
:
"message body in CONNECT request"
Closed issues:
:
"Default charsets for text media types"
:
"Content-MD5 and partial responses"
:
"untangle ABNFs for header fields"
:
"confusing undefined parameter in media range example"
Closed issues:
:
"Clarify status code for rate limiting"
:
"clarify 403 forbidden"
:
"Clarify 203 Non-Authoritative Information"
:
"update default reason phrase for 413"
None.
Closed issues:
:
"Strength of requirements on Accept re: 406"
:
"400 response isn't generic"
Closed issues:
:
"Strength of requirements on Accept re: 406"
Closed issues:
:
"Redirects and non-GET methods"
:
"Document HTTP's error-handling philosophy"
:
"Considerations for new header fields"
:
"clarify 303 redirect on HEAD"
Closed issues:
:
"Document HTTP's error-handling philosophy"
Closed issues:
:
"Location header field payload handling"
:
"Clarify status code for rate limiting" (change backed out because
a new status code is being defined for this purpose)
:
"should there be a permanent variant of 307"
:
"When are Location's semantics triggered?"
:
"'expect' grammar missing OWS"
:
"header field considerations: quoted-string vs use of double quotes"
Closed issues:
:
"intended maturity level vs normative references"
Closed issues:
:
"Combining HEAD responses"
:
"Requirements for user intervention during redirects"
:
"message-body in CONNECT response"
:
"Applying original fragment to 'plain' redirected URI"
:
"Misplaced text on connection handling in p2"
:
"clarify that 201 doesn't require Location header fields"
:
"relax requirements on hypertext in 3/4/5xx error responses"
:
"example for 426 response should have a payload"
:
"drop indirection entries for status codes"
Closed issues:
:
"is ETag a representation header field?"
:
"Content-Location doesn't constrain the cardinality of representations"
:
"make IANA policy definitions consistent"
Closed issues:
:
"should there be a permanent variant of 307"
:
"clarify that 201 can imply *multiple* resources were created"
:
"merge P2 and P3"
:
"ABNF requirements for recipients"
:
"Capturing more information in the method registry"
:
"note introduction of new IANA registries as normative changes"